Colorism is a type of discrimination that is a step beyond racism, where people are marginalized not solely due to their race but because of the literal shade of their skin. The coining of the term is widely attributed to the author Alice Walker. People are treated differently for having darker or lighter skin, even if they are of the same race. And often that discrimination comes from people of that same race.
Image taken from this website.
Slaves with lighter skin are assigned tasks inside the house while slaves with darker skin were forced to work outside in the fields, doing much more grueling tasks. Lighter-skinned slaves were favored because they were often the product of a slave owner raping a slave, thus creating a lighter-skinned child.
In the 19th and 20th century, the "paper bag test" was often utilized in black spaces and in hiring of black people. If someone was the same color as or lighter than a paper bag, they would be allowed into the certian spaces and considered for roles. If they were any darker than a paper bag, they would not.
Skin tone was often the most important factor in applying for work as a person of color in the mid-20th century. Light skin was often reported on a resume ahead of any other information or experience.