I just make them up. They don't come "at the drop of a hat"; it takes time and a lot of hard work, usually. Often there are false starts and wrong turnings. One site design I did a while back went through four utterly different designs before I was happy with it. Sometimes that's just how it is; usually things go a little more smoothly.

Usually it's art that I draw from, rather than the work of other designers--Rothko, Hokusai. Recently I saw an understated gem of an exhibition: Vilhelm Hammershøi at the Royal Academy. I would love to incorporate some of his subtle tones and textures into something.