Here are a few dictionary definitions of colourism. I don't want to delve too deeply into definitions and I prefer to focus on the deeper social issues. One point I should make quite early on is that I don't generally agree that it is mostly down to European colonial influences in Africa and Asia and the slave trade in the Americas.
Discrimination against people with a different skin tone, esp within the same racial group
Discrimination based on skin color, also known as colorism or shadeism, is a form of prejudice or discrimination in which people are treated differently based on the social meanings attached to skin color.
Colorism is a form of racial discrimination based on the shade of an individual's skin tone, typically favoring lighter skin. It can occur both within a specific ethnic group and across ethnic groups.
To add more colour to a boring looking blog here is the delightful and effusive Pratyusha Pilla giving a talk at TED on Colourism