Precedent states that british english is used, and thus "colour" is preferred in settings? Internally, and I am partially responsible this, however, "color" is used interchangeably. It might be nice to replace all appearances with "colour", or vice versa in the code base.

I'd say go for "color". As a non-native English speaker, I know I tend to mix e.g. "behaviour" and "behavior" frequently. But when it comes to programming, "color" is kind of well-established. For better or for worse, I think we should try to stick to "simplified English" instead of "traditional English" as defined by Steam.