In every image I create, I take time to consider which colors are strongest/most complementary to each other, and to the captured scene/mood. Having a good understanding of how complementary colours work, and how they help to enhance the desired mood in an image, can drastically improve the overall impact of our imagery.

This wonderful tool from Adobe will allow you to pinpoint which colours work best together:

And this excellent tutorial from The Art of Photography will give you a good grounding in colour theory and a better understanding of how the tool works.