Here's some small colour thumbnails I did. I'm forcing myself to work on these small without zooming in, trying to keep it simple and just use the right colours and values to make them work without rendering. It's a really good exercise, and when you do lots at once you really start to notice the compositional and lighting cliches you fall back on. These are a great way to explore ideas for bigger pieces, if they work this small they will be great in larger sizes...you've also got a pretty complete colour palette after an hour or so.
And here's a sketchpage...
