Filters

You could buy Mig's German camo filter set. It has three bottles of tinted, solvent-based filters for Panzer Grey, Dark Yellow/Red Brown, and Olive Green. Basically you get a blue filter for the grey, a greenish filter for the olive, and a reddish-yellow-brown filter for the Dark Yellow/Brown

Myself, I just use dabs of red, blue, green, yellow, peach oil paints and work them into the model's surface with Turpenoid (blue bottle). You might practice on an old model or airbrush some colors onto a sheet of styrene to mess with first.