And dark green like they have in some Australian prisons.
And a couple of other good ones would be, light brown, dark brown and white.
And like what @Bigurn said with the stripes, maybe add in black/white, brown/white, red/white, green/white and grey/black just to name a few.
And maybe add in an option to give your staff and guards different styled uniforms aswell, I think this would be a great addition to the game!