I just read a really interesting blog post on color theory from the perspective of a former Hanna-Barbera background painter by the name of Art Lozzi. Examples of his work on the Yogi Bear cartoon are used to explain his tips on using color via the tools of the time: namely brushes, sponges, chalk, etc. I found the bit about a main background color (he chose “salmon” because Yogi looked good on it) peeking through other layers particularly interesting:

You can see the peach sky in the little holes in the hills where the sponge didn’t paint. This gives the effect of mixing the main BG color with the other colors in the BG and it ties them all into one harmonious color scheme.

This sort of “layering” to create a unified theme is certainly made easier these days with Photoshop, etc. (as pointed out in the Drawn! post that led me to this). Great stuff.