There are stereotyped object colors that are highly prevalent in fiction, but do not reflect Real Life color variations. They have become embedded in popular consciousness through sheer repetition, and sometimes Editorial Synesthesia, to the point that exceptions just seem odd.
Subtrope of Reality Is Unrealistic. The color version of The Coconut Effect. Supertrope to Bicolor Cows, Solid Color Bulls, Green Gators, Red Live Lobster, and White Bunny.
Examples:
Purple Grapes
In Real Life, grapes can be blackened dark blue, yellow-green, red violet, and red, but in fiction, they're purple because purple is the color grapes are associated with as well as being the color of Cabernet Sauvignon, the stereotypical red wine people always imagine. Green might be used, but it would give the impression the grapes are underripe.
Medium-Blue or Light Blue Water
In Real Life, water comes in a wide range of colors, including clear, turquoise, cyan, light blue, blue-green, dark green, dark blue, and the stereotypical bright shade of blue.note
Not so much in fiction, where water is usually a bright shade of blue. Water Is Blue because it's easier to animate than a transparent liquid and because large amounts of water appear blue due to the way it diffuses light. This subtrope, which is highly prevalent in fiction, does not reflect how Real Life water is entirely accurately.
Blue Pluto
Back when it had its planetary status, Pluto was often depicted as blue. This most likely had to do with the cold nature of the stellar body, but was probably helped by being near the truly blue Uranus and Neptune. It wasn't until photos came back from New Horizons that Pluto was revealed to be a lovely shade of peach (much closer to the color of Mickey Mouse’s dog of the same name). People on social media invoked this trope by (often jokingly) insisting the dwarf planet was blue.
Green or Yellow Green Acid
It even has a shade named after it
despite the fact that green acidic substances are rare, and no strong or commonly used acids are green (in fact they tend to be transparent as they are usually most active in when in watery solution).
Green or Yellow Green Grass
In fiction, grass is usually green or yellow green because green or yellow green grass is iconic and easily recognizable. What people are usually thinking of is "Kentucky bluegrass", which became a status symbol for front lawns of suburban homes in the 20th century.
In Real Life, golden yellow, light brown, and sandy yellow, as well as the stereotypical green and yellow green, are common colors for grass.
Green or Yellow Green Radioactive Nuclear Waste
Green and yellow green are the colors associated with nuclear waste, radiation, and anything nuclear even though this is seldom the case in Real Life. Cherenkov radiation in the pools of nuclear reactors is blue
◊, radioactive cesium chloride fluoresces faintly blue, and hot radioactives are orange
◊.
The association with the colors green and yellow green and nuclear waste comes from peoples' experience with radium-painted watch dials and uranium glass, both of which which glow pale green (the former in the dark and the latter under UV light). While uranium glass is still produced in small quantities, watch dials haven't contained radium for decades. Currently they use a nonradioactive phosphorous paint that absorbs light when placed in it and then glows for a while in the dark.
Yellow Sun
In Real Life, the sun is white or yellowish white, but in fiction its yellowness is played up because yellow is the color associated with the sun. Part of the confusion is likely because the sun is classified as a yellow dwarf star. That said, stars have very weak colors in general. "Red" dwarf stars, for example, are more of a dark orange.
Yellow or Gold Stars
In Real Life stars can be one of several colors (see below), but the stars the naked eye can see in the night sky are white. In fiction, stars in the sky—especially those drawn as large, five-pointed "sticker" stars—are usually yellow to contrast the night sky's dark blue.
Star-shaped stickers and badges are virtually always gold in fiction because they're given out in a congratulatory manner—in other words, they're a proxy for a gold medal. Needless to say, in Real Life you can get star stickers and badges in pretty much any color, though gold is still the most popular.
Throughout the universe, stars can be blue, blue-white, white, cream, yellow, orange, or red; this depends on their temperature, with hotter stars being bluer. This can be confusing to people who associate red with hot and blue with cold!
An interesting inversion of this are neutron stars, which in many illustrated astronomy books are shown as being red. In reality, due to their very nature, they are invariably white.
Black Night
The night sky is almost always depicted as solid black in cartoons and comics, while in reality it's usually a really dark blue, due to the stars and moon (if it's out) providing at least some light. The term "midnight blue" exists for a reason.
Yellow or Orange Cheese
Cheese comes in many colors, including yellow, orange, yellow-orange, light yellow, red, white, and even blue, but in cartoons, it's hard to find any examples where cheese hasn't been depicted cheddar yellow-orange or orange.
Bright orange cheeses, especially cheddar, are usually that color because they have been artifically colored.
Orange Carrots
Almost all carrots in fiction are bright orange because an orange carrot is iconic and easily recognizable and orange is the most common color seen in Reallife Western carrots. note
This association with the color orange and carrots is Newer Than They Think, as the common orange carrots in the Western world were only bred that color a few centuries ago by farmers in the Netherlands, out of patriotic reverence for the House of Orange-Nassau. Before this, most carrots in the West were actually purple. No, really, carrots can be purple. And carrots come in other colors, like yellow, red, purplish red, and white
◊.
Red Apples
Most apples in fiction are bright red because a red apple is iconic and easily recognizable.
In Real Life, apples come in a wide range of colors, including red, orange, yellow, and green, and can be more than one of those colors as well.
A common exception would be "sour apple" or candies that are apple flavored, which are often green, especially to distinguish other flavors, such as strawberry or cherry, that are almost always red.
Red Fire Hydrants
In Real Life, they come in a wide range of colors, including yellow (both light and golden shades), pink, white, dark green, orange, dull shades of red, and the stereotypical bright shade of red, and can even have two or more colors on them (like San Francisco’s blue and white fire hydrants).
Not so much in fiction, where almost all fire hydrants are bright red. Fictional fire hydrants are usually red because a red hydrant is iconic and easily recognizable. This subtrope, which is highly prevalent in fiction, does not reflect how Real Life fire hydrants are.
Similarly, fire engines/fire trucks are almost always red in most media to the point where some shades of bright red are informally referred to as "fire engine red". While red is still the most popular color for fire-fighting vehicles in real life, some cities and departments have painted their trucks yellow, white, or fluorescent chartreuse for higher visibility.
Brown or Green Polluted Water
Water intended to look polluted is colored either olive green, yellow-green, or brown rather than blue or clear.
Same concept is also commonly applied to swamp water, "blackwater" in real life can look brown, tan, greenish, or pitch black.
As a side note, just because water is green or brown doesn't necessarily mean it's polluted: water could be green from algae or brown from mud or silt. Conversely, as noted at Cool, Clear Water, just because it's clear doesn't mean it's not polluted or host to dangerous microorganisms.
Brown Coconuts
In Real Life, coconuts come in either green or yellow, turning brown as they ripen.
In fiction, namely Western works, coconuts will usually be brown due to the brown and hairy coconut shell being more familiar. When immature coconuts do appear, however, they will always be green.
Sand-Colored Pyramids and Sphinx
During Ancient Egypt, pyramids were originally covered in limestone and gold, making them white with a gold tip. But over the years, the gold was stolen and the limestone was taken for the constructions of other buildings in Cairo, thus giving the pyramids the sand-yellow color they have today.
The Sphinx was also originally brightly-colored; traces of pigment suggests it had a red body, a yellow and blue headdress, blue eye markings, and a blue beard (now broken off). Its now sandy coloring may have been a result of burial and erosion over the millennia.
However in fiction, both are usually depicted as sand-colored regardless of time period.
White Bones
Bones will usually be colored white, which is somewhat realistic as bones tend to be white or yellowish-white due to the high amount of calcium. However, bones do yellow out and darken over time, and indeed excavated human remains tend to be dark amber in color even if it's only been a few decades since they've died. Because of this, skeletons donated for scientific display typically have to be bleached to achieve the more familiar snow-white color, and plastic skeletons manufactured for school science classrooms and Halloween decorations are hardly ever made in any color but white.
This even extends to the fossilized bones of extinct dinosaurs and other prehistoric animals, despite the fact that fossils are remains turned to rock. Consequently, the skeletons of prehistoric animals in museums will also be white as well, since people tend to forget that fossils on display are actually replicas of the real fossils (kept shut away since most of them are fragile and/or too heavy to arrange in life-like, active-looking mounts). This may be starting to wane somewhat as more modern depictions of fossils show them with a dark gray-brown or black color more reflective of their real-life counterparts.
Skin Color
The skin colour of white people is nearly always depicted as pink, and we're told it's pink, but in reality its a much more subtle selection of light browns, translucency, red beneath the skin, with some blue. It's also often depicted as the stereotypical "Celtic" skin tone: very pale pink, turning bright pink or red when sunburned or after strenuous exercise, when many people of European descent are at least a little darker. Caucasians with darker shades might just be colored brown out of sheer laziness (see Phenotype Stereotype).
Black people in fiction are always milk-chocolate brown, or even mahogany red. In real life, people who self-identify as "black" can vary in color from pale (beige) to very dark (seal-brown) due to natural morphological variation in ethnicities, but this range is rarely depicted (the fact that seal-brown skin was a common feature of blackface and all the racist overtones that came with it certainly doesn't help). Aboriginals of Australia and East Indians have skin tones with a similar shade variety but they are also exemplified by a middle shade of brown or reddish Brown. Giving a character a top square "Starbucks" complexion, meanwhile, may make their ethnicity ambiguous (light black, dark white, or beige Asian).
Asians run the whole color spectrum, from very light-skinned Manchu-Tungus peoples of the far north to dark-orange or light-to-dark-brown Austronesians in South/Southeast Asia (Indonesians, Filipinos, etc.), to say nothing of Afro-Asians who are commonly dark-skinned and frequently, as the name suggests, mistaken for black Africans. East Asians have often been stereotyped as yellow.
Yellow Urine
In real life, the urine of a healthy, properly hydrated person is usually a very pale yellow; almost clear. Darker yellow urine typically means the person is not getting their recommended daily water intake, as water dilutes the yellow pigments in urine. However, completely clear urine typically indicates that you've been drinking too much. People with darker yellow urine, and there are many of them because of the large number of people who don't like drinking water (though to stay hydrated, any liquid will do, from hot chocolate to guava juice, but some drinks shouldn't be drunk in excess), typically assume everyone's urine is that shade of yellow, causing this misconception to persist.
Green Snot
Any character with a runny nose will have bright, yellowy-green snot regardless of the reason. In reality, green snot is a sign of infection, while allergy snot is white or clear, sometimes with yellow flecks in it, and is dark yellow when dried.
Pink Gum
Most gum in fiction will be bright pink. In reality, though pink is common, gum comes in other colours too. Gumballs seem to be the main exception to the rule, if only because bright, multicolor gumballs have been a real-life mainstay in western society for ages.
White Or Clear Diamonds
Diamonds in fiction are almost always depicted as the standard white/clear color. In reality, they come in a wide variety of colors, and the colored kinds are often more valuable than the clear kind. Part of this may be Rule of Perception: people might mistake the diamond for something else if its a different color.
Blue-printed blueprints
Building or invention blueprints are always shown in actual blue printing, despite that reprographic process being obsoleted in the forties and its replacement being also obsoleted by modern photocopy. Sometimes, the blueprint is implied to be original instead of copied, another reason it shouldn't be blue-printed in the first place! It's like the architect actually used a white pencil on blue paper to draw it. note
