i don't get the concept of confining races to only certain classes let alone limiting what colour your character can be because of their class.
Well it wouldn't really make sense to have a race generally not strong in the force playable as strong force using classes turning the "Very few can really use the force" into "Oh look, there goes a conga line of 'em all with strong force usage.".
Same goes for races that may generally be lacking in the muscle department. A class like trooper involves bulky armour and very heavy and cumbersome weaponry, so with that in mind you couldn't really make a ewok into a trooper for example without recreating the entire race and telling George Lucas that his lore is wrong now.
As for skin tones, it's quite plausible that for some species skin tone is based on some chemicals produced by the races' body and that one gender will produce a different combination of chemicals (or just different concentrations) than the next.
Humans aren't all that different to this in some ways. Our bodies are shaped and grow differently not just based on how active we are but also what's going on inside on a chemical level.