I wanted to expand a bit on my post yesterday about mocking up the colors in GIMP. Basically, I take a fairly transparent paint brush and paint in layers. Usually each color block is a layer, but this time I got lazy (sword, armor and lights are all on one layer).
Here’s a couple (very) quick examples of how flexible this is. Once you’ve got the brush paths saved, you can manipulate the hue/value/saturation/etc. GIMP is a bit obtuse and I’m not very good with its tools, but for rough prototyping you can see where it’s handy.