I had a ook at the Paint.Net documentation, it doesn't seem to have what I'm looking for.
You see where there are pale grey chains on his belt and the jeans are blue? The grey is on the top layer and the blue is on the layer below, but every time I try and colour behind the chains, it darkens them instead of leaving them grey. Both layers are set to multiply so that I can see to colour, and I think the problem is that Paint.Net is reading anything that's not absolutely black as translucent, so I guess my question is, how can I get opaque, strong lines and still be able to see the layers below?