I think it is because people do believe that said colors are more like guidelines, instead of being something you must follow. I see little issues with people coming up with their own variation of the base colors, as long it is not something like a black m'sai or a the like. For example, among unathi players, there is no real RGB indication of what unathi can be, and we rarely have issues with players being dumb with colors or something.
I don't see much reason to enforce people to follow said colors, as long they keep in the range of them, follow the tajaran ethnicities theme and don't do something dumb (which are the minor cases here), there is little reason to do that. Besides, any people doing something really dumb could be handled like other regular whitelist infraction.
So, I think things are fine as they are. I have no trouble in figuring out the ethnicity of most of our tajara's character just by looking at them, I feel that enforcing everyone to follow sanctioned colors is not needed, because, there is little to no problem with that, the tajara players are not really ignoring this aspect of the species.