I'm no painting expert, but I use multiple colour washes. Some of the uses I can think of are:
Green wash: Great for shading Greenskins (over goblin green) and Nurgle models (over rotting flesh).
Blue wash: Good for magical armour (if kept light), blues or purples.
Red wash: Good to coat anything that's meant to be bloody or just deepen red cloth.
Brown/flesh wash: Good for skin tones and other browns.
Black wash: Best on metallic armour and weapons. Also works on flesh tones to create a 'dirty' look (I used it on my bretonnian peasants that way).
I have never tried the idea of opposite colour washes, but it sounds interesting!
"The battlefield is a scene of constant chaos. The winner will be the one who controls that chaos, both his own and the enemies'."
- Napoleon Bonaparte