I recently finished painting the first two models for this project: a shoota boy and a slugga boy.
I wanted to stay close to the aesthetic of the Orks from the original Dawn of War I PC game. In practice, this meant a drab olive green skin, plenty of weathered and dirty metal, and the occasional pop of bright color as an accent.
The primary colors for my Ork army are going to be bright red and yellow, which I thought would contrast nicely with the dull green skin and muddy greys of the urban landscape I have planned.
Here's how the test models look after two coats of Liquitex gloss & matte varnish:
I think they turned out about as well as I had hoped!