This is the L*A*B* color system. Colors opposite of each other will bring out the color of each other. So back to out friends awesome Blood Angel Captain, if he had put it on a base with maybe some grass like a rocky base as long as there was some green grass it would of made his Red Captain pop out more. Or even maybe some swampy green moss or sewer water anything with a green. Now another thing to remember is beware brown. Brown can suck the life out of all color if you use to much. But a good mix of that and green it good as the brown will drain the green, and the green will still empower the red.
The other side of this say you have a army with a camouflaged theme like Catachans. You may want to have a very earthy green and brown base to hide them in it. This can add to the overall feel and blend of the army making it ten times better looking.
Basing is a huge way to tie your army together. On top of that thew potential to making your army's color pop more is astounding. So when you decide what color scheme you want, you should then think of how your going to base them.