At this stage, colour choices don't really matter. I change my colours often as I work, and alot towards the end of the piece. For your shading just follow the steps, decide where the lightsource(s) will be, block in shadows, block in highlight. Iterate with more tones as required. Tint tones with desired colours. Apply texture. That's pretty much how I do it, anyways.
Actually I'd recommend to start you paint it all in just 1 tone. I use a mid grey of about 38 value/brightness. Colour everything that, then pick a dark grey of like 20, shadow, etc etc as I said above. Concentrating just on values at the start keeps you focused on nailing the shading without getting distracted or thrown off by colours.