I tend to work in a different way to what I mentioned on here. I create a new layer, set it to multiply, and colour it in a vivid colour (such as red). I then work into the layer with the various colours, using a brush tool rather than the selection tool (which I mostly use for making grey knock-back layers). I guess this comes from my (now distant) background in painting, and it creates a rather nice effect, with the background canvas colour sometimes sneaking through in the odd place here and there to provide some extra texture.