When I rate an MoC, I not only judje its design, but also by color. Here's what I find interesting:

-Red goes well with yellow, orange, and if you want to make it volcanic, add a little grey. Gold is also good.

-Blue goes good with "glass" or grey. Silver is a good exception of a metallic color.

-Green and grey is a good combination. Gold may look okay on it as a metallic touch.

-Black is good with white sometimes, for I like Yin-Yang colors. Gunmetal or Silver is good with it.

-White is good with black (see above), and with Thok blue or "glass."

-Brown works well with yellow or gold.