Why would a 'bad' grymn faction have to have a different coloured skin?
I'm pretty sure that most races would have differences of view on politics, religion, science, ecology and the right end of a boiled egg to eat first. All of these subjects have certainly been known to cause wars between humans (and in one case very little people) so why does it have to be different in grymn? I'm loathe to point out some of the current wars on our own planet as evidence; I'm pretty sure no-one would admit to their own forces being "the bad-guys".
One of my pet hates in Fantasy and Sci-fi writing is the way whole races are characterised together in homogenous blocks with little variety, whilst humans continue to portray a wide range of beliefs, habits and behaviours.
There are clearly 2 exceptions to this rule however.
All dwarfs are loyal, courageous and right.
All elves are efeminate, pointy eared, interering self righteous bigots.
The only good orc is a dead one.
Ok, so there are threee exceptions...
(Stunty and proud)