I think you might want to refrain from colouring your drawing for the moment. If you don't, you might end up creating shading that will look fine on one camouflage and completely wrong on another. If you do this, you'll have to start over the shading, which is generally one of the toughest things.
I start with solid 50% grey. It's not perfect and needs twitching once you're finished but I think it's a more convenient and safe way to work.