Chi accepts
colorism, as a rare word, and always has. Oddly enough,
colourism is not currently accepted at all. This probably reflects the fact that the term originated in the US, so the American spelling was probably the only one given in the source dictionaries from several years ago that were used in compiling the list I started with.
Wiktionary defines
colorism/
colourism as:
Prejudice or bias against persons on the basis of their skin color or complexion, often among persons of the same racial identification.
As noted in the
Guardian article you mentioned, MK, this prejudice can be exhibited by people from the ethnic group concerned or it can be by people who regard themselves as white - e.g. lighter-skinned slaves could work indoors while the darker-skinned had to work in the fields.
The word can also refer to artistic styles, but in relation to skin colour, the OED has a 1964 citation where the word simply meant racism, but the earliest example relating specifically to skin tone was from 1977:
Racial discrimination in Brazil has been described as 'colorism'; in this case, the lighter one's skin, the higher, socially, they may rise.
The News on the Web corpus shows use of the word, in both spellings, has grown rapidly over the past few years. The majority of occurrences of
colorism are from the US, but use of
colourism comes from many countries in Africa, South Asia and the Carribean, as well as the UK and Canada.
Probably we should add the
colourism spelling to our lexicon.