There are certain realities of physics that determine how light and values work. The laws of color are a series of algorithms that determine how light moves through space, and then we have a set of biological limiters that define our ability to perceive that light. Our eyes and brain are designed to see only visible light, which is a tiny band of wavelengths in the Electromagnetic Spectrum.
Working with color, you are moving in two worlds simultaneously. The world of visible light, and the physical world of whatever your medium is. The rules of color harmony are constant, like light itself. But light is a vast subject, and thoroughly amazing in its power. Playing the numbers of physics with the expressive quality of art is what a good colorist must do.
Every painter who ever lived was a colorist. How they approached their craft was up to them, but the physics of the rainbow, and how that rainbow lights the world, determined all their techniques.
We deal with color harmony every day the sun comes up. We're treated to a smorgasbord of colored stuff just moving around. All around us is light bouncing off things and chemical processes percolating. It's the Quantum Stew. That's where we live.
My home is in the country near the Pacific Ocean. The light here is spectacular. We have sunsets over the water, which I think are the best. The light bounces off the water and gives the clouds a little underlight fill. It's something landlocked places don't get. Point Arena is one of the farthest points west in the continental USA, so we also get vast cloud formations, which always make a sunset better. The skies here are alive with winds fresh off the ocean, and those clouds do hard-to-believe tricks that I'd never be clever enough to make up. We have great color skies here.
Wherever you are, your sky determines your light. What time of day, what time of year, and what the weather is sets the overall color tone, and from there, you just work out the light and shade of your subjects. It's a pretty elementary process, except that it gets subtle when you start trying to mix colors, and try to duplicate reality in paint, pencil or computer.
If you're trying for a form of Realism, then you must obey the laws of natural light. If you are using an Expressionist approach, you'll follow some of the rules of color harmony, but you can ignore all the conventions of accurate modeling. And if you're going for the Egotistical Artist, you can ignore all rules and take your chances.
Once you know which of these philosophical directions you're going, (which includes being aware of all the physical details of paint type, pen type, paper, size, resolution, film output, computer, etc) you can begin making decisions about your project.
When I color a comic book, I begin by determining the mood of the story, and the flow of time through it. At the same time I'm figuring out character and location colors. (Sometimes that's already determined by the artist.) I'm a realist for the most part, so I next make some rendering decisions. I figure out where the light is coming from in each scene, and then begin coloring. The rendering and execution of color theory that comes next is what makes the difference between color art and color craft. I've done both. I've done some very fine coloring, and some journeyman work.
The real difference comes in how much you care about the particular art and story you're working on. That, and how much time you need to make something come out right.
In conclusion, though, the old saying is as true as ever:
"Even great color can't save bad art."
So start with good art if you can, it really makes everything easier.