Remember, Orks are a species(?) of fungus. Fungus doesn't need chlorophyll. So it's not like there's even a good reason for them to be green.
The other Orks might give them a hard time, but a punch in the teeth solves everything.
Automatically Appended Next Post: As for the paint thing; are you using like normal house paint?
Personally, I'd stick to model paints. Find some Vallejo paints (chaosorc.com has them), they come in little squeezebottles that are convenient to use, and you don't need a whole gallon of it.