Dark-Self is an evil version of a character. This can happen in different ways; such as them geting brainwashed, trigger(s/ed), alternate events/timelines, etc.
The Dark-Self versions usually have darker or duller colors than their "good" or former counterparts, but this is not always the case. Sometimes alternate colors can be used; like light pink to [a shade of] purple, instead of light pink to a darker/duller pink.