Color dying poly frames?

Ok, here's an odd thought: how would a light colored polymer frame be dyed a darker color? I've seen the Steyr AUGs, HKs, and a few other rifles dyed darker. How is it done? Reason: I love the Walther P99 (in .40) and I've been eyeballing the sand colored P99s but think they're way too light. Random thought crossed my mind; 'two tone camouflage in OD/Sand or Brn/Sand or X/Sand...' I also have an XD40 in med green that would look interesting in blk/grn or gray/grn. So, how's the dye done on polymers, and would this be feasable?

SEM dye.

Google it.

Its available here in town.