It might just be easier to point you to a couple threads.
This one is Fallout related. Every mini had this done to some degree or another. Mostly the skin, but also the harder bits of the Deathclaw and the Power Armor.
For non-Fallout related, here’s a Work in Progress thread I did for another game where this was used on skin, clothes, and fur.
http://www.coolminiornot.com/forums/showthread.php?67683-Alxraven’s-Night’s-Watch-WIP-Thread
It’s important to have a good idea of what the wash is going to do to the colors you use. Then pick colors that will end up where you want them tone wise, after the wash is applied. In almost all cases, you’ll want to go with something lighter than the result you want.
For example, a dark wash over blue will darken the color. A blue wash over the same blue will deepen the color, and make it even more blue.