I honestly don't even know the true depth or meaning of color theory. What I know I have learned from carrying plants around nurseries comparing harmonies and contrasts. I know what looks smashing together and what hurts the eyes from trial and error.
Some of my first experiences working with color, and also values, was looking at antique Amish quilts and then making a few miniature old-Amish-style quilts . The makers of these quilts were true artists. They knew what looked delicious by laying the colors next to each other. They often had just one or two colors, but different shades of it, set against a dark or black background. The dark background gives depth.
One of my favorite "rich" color combinations is red and orange. Then add in burgandy and wow, it's a knock out.
I started my very first garden bed with annuals. Then I moved to perennials and sniffed at annuals feeling I had, well sort of, graduated to the next level. I still had yet alot to learn about gardening. It is the art of mixing the perennials with annuals and shrubbery, not just colors, that makes you a master.
If you stay right with the colors that rub shoulders on the color wheel and then make them smokey, you can't go wrong. True bright colors tend to clash and hurt the eyes. Smokey colors are restful. For every rule of thumb there is an exception it seems. Other restful colors are soft yellow, hazy purple blue and gray. Think foxgloves, perennial geraniums and artemsia. Blue is not next to yellow, but yellow is in the making of the color blue. So, it's going to look good.
White is always a color in the garden where the eye can relax. Mrs. Hobhouse put it in her red garden. Unless it is a running through the flowers white westie. Sir Salty thinks that he is the dustmop and must touch all plants while going over and going under them. Then he goes from white to gray...which is restful... (if you are a plant color)...but not when you are a white westie dog. Then you must get right in the bath again.
My doggy boy loves toys and often picks one up and carries it to where he lies down and takes a quick nap. Sometimes he puts his head directly on it --so no one can take it away-- other times he puts it right next (and I mean right up against his eye) to go to sleep. Maybe I need to buy him shades? And his own carry along brush?
If you have a great picture of your own plant combinations I'd love to see it. There seems to be no end to mixing up something wonderful.