When it comes to learning to paint, this is one of the most valuable lessons you will learn. Using complimentary colors to work for each other. I'll try to convey this in the simplest of terms...
Compliments are as follows...
Orange - Blue
Red - Green
Yellow - Purple
Compliments mixed together create a grey, u can go warmer by adding the warm colors or make the grey a bit cooler by adding the cooler compliment.
If you find the red or green is too intense then add its compliment to tone it down. if you want a highlight to really stand out then use its compliment to set the stage.