I was hoping you'd know of a useful tool - I actually used to use the army painter in the first Dawn of War, but the later ones (and space marine 2), although allowing a greater range of colours to play with, have much more limited units....

Ah I'm sorry, if anything I'll often just have the product page for what I'm painting open on my phone so I can refer to that, if I'm going with the official schemes. But other than that I really do just wing it!