I've seen them painted silver, looks like a lot of wort that was hardly noticed.
I've seen them painted black, those kind of get lost under the vehicle.
I had a 4x4 unit painted blue, looked really out of place against the orange small block in the truck, but it was a great trans, 12 years of plowing snow without an issue. It got broken when the truck got wrecked.
Painting a trans is a lot of work for something pretty hard to see most of the time, imho.