A lot of this problem stems from colonialism (as mentioned in the main entry). The modern iteration of this is caused by the media's interpretation of beauty, and their Eurocentric standard of beauty, which unfortunately causes a lot of unfair resentment and friction toward light-skinned blacks. It arguably also caused a lot of blacks to exploit this to their benefits to the chagrin of some dark-skinned blacks. Whether we like it or not the favoritism is there. This can be witnessed first hand for very fair-skinned black women.
What has always been confusing about this is what is the defining line between light skin and dark skin? What about people who are in the middle of the spectrum?
It's a matter of degreeism, and probably phenotype.
Historical causes other than colonialism
There are also other causes than colonialism. In feudal age societies all over the world (not only in Europe, but also in many Middle-Eastern countries and China, Japan and India to name a few) darker skin was associated with year-long outdoor peasant labor, whereas fairer skin was associated with nobility kept indoors (and it wasn't limited to skin tone, but encompassed other attributes of labor, like muscles and being skinny). Pale women in particular were held as more beautiful. Of course, in Europe, both North and South America and South Africa this was then compounded by the many consequences of colonialism and slavery. But one could also wonder if the preexistence of this trope is not the main reason why native Americans were deemed improper for slavery by the Catholic church whereas darker-skinned Africans weren't.
The most egregious example would be today's India. You wouldn't know it from TV, since most Bollywood actors (and US actors from Indian descent) are higher caste and lighter-skinned. People from the Indian sub-continent come in a much higher variety of skin pigmentations than seen in movies. Castes in India in turn come from a long, long way back. Yet, skin tone is highly correlated to caste. And castes are in turn highly correlated to revenue, health, life expectancy, etc. Indians at the lowest caste, Dalit or untouchables, tend to be much darker skinned, whereas many higher caste Brahmins would pass as white in the US. India, of course, never owned a colony in Africa and only very marginally if ever used Africans as slaves. But then, here also, Western media has accentuated preexisting biases.
Practical reasons for the trope
In modelling and live action it might (at least partly) be that people with fairer skin are easier to photograph and film; without proper lighting, a black person gets reduced to a black human shaped blob with visible eyes and teeth, whereas the facial features kinda disappear. Not to deny that other factors can be behind this, espescially the more social aspects of the trope, but the more practical aspects of it deserve a mention, as well.
This has more to do with the photographer not knowing what he's doing, or not adjusting the lighting on the camera to photograph someone darker-skinned. When black tropers think back on the various picture IDs they have, the ones taken in cities and schools with a healthy black population tend to turn out much better than in places where they're the only black person for miles.
Definition of "black"
This trope may also be related to the tendency to treat blackness as a "dominant gene" — i.e., people define blackness in such a way that if you have one black grandparent and look like it, then you are black. This results in an expansion of the set of people who qualify as black, and it's also relevant that mixed-race blacks are more likely to come from a middle-class background.
That definition is pretty unique to the US, and while it is the third most populated country in the world, only about 12% of it are traditionally considered black. The definition of black can vary by a lot depending on whom you ask, as well as any perceptions being any "kind" of black brings.
It stems from the American "one drop rule" whereby if a person had at least some black ancestry (how much legally varied) they were considered black (even if their features were white-passing) with subsequent lesser rights (or even being enslaved). However, even in the US various categories of mixed race people were made as well based on how much black ancestry a person had (one parent, two grandparents, one grandparent etc) while obviously the less they had the better, legally and socially, for a very long time. Even then though in some cases mixed race "free people of color" were sometimes privileged and wealthy, some also owning slaves themselves (though obviously they were the exception). This mostly occurred in Louisiana, which had the French customs passed down that were somewhat looser than later American ones regarding this.
How do class play a part in colourism/intra-racism?
Noticing that most people like to bring up their social standing when discussing race. People seem to think all fair-skinned blacks are educated upper middle class law abiding citizens, and darker blacks are lowly uneducated low class criminal ghetto dwellers. Forget how white people view us, how about how other blacks see us?
This perception dates back to colonialism and slavery. Biracial blacks—often the products of rape—were usually given cushy indoor jobs. If they were free, they were more likely to be allowed to pursue apprenticeships and have a career beyond menial labor. And if they could pass as white (say, if one parent was mixed and the other was fully white), they would have just lived as white with nobody being the wiser. This is more pronounced in Latin America and South Africa, where mixed racial groups were a distinct class with some of the rights and privileges afforded to whites (as opposed to the USA where either you were black or you weren't). As distasteful as all this sounds, it's a legacy that lasted centuries, so it's not surprising that the idea persists into the present day.
