Color theory does to painting what grammar does to language. It all depends on how colors complement or contrast. It is a fundamental need and a tricky one. Sometimes the only difference in an 'awe-inspiring painting' and 'could be better painting' is just the tuning of colors that lack the unerring proportion.
There are tools available in the market to ease out color mixing. But getting hold of the art of mixing colors is a long-time investment and must be given a shot before.
Tips on color mixing:
Take a photograph: While painting live human portraits or scenic scapes, mixing colors take a lot of time. For beginners, it can be one of the main causes of painting getting ruined. A photograph brings the scene on the table and can stay put right next to the palette to mix and match the shade that will go on canvas.
Color of light: Light is not of the same color all the time. There are days with orange mornings while the others color them pink. When nitrogen gets all playful the sky becomes green and a few days rainbow shows up. Ever wondered what will happen if you mix the shade of light to the color you made. Try it!
Avoid understocking or overstocking: This is useful for beginners. Pick the colors after you make your mind on what you will be painting. This way you won't be stuck with the pressure of using colors that you have before investing in new ones.
Mix with Palette knife: Mixing colors with brush may trap color blobs in the brush hair and might end up giving an entirely different color than what you mixed on canvas. Palette knife overrules such mix-up.
Choice of palette: Choose a color palette that has space for colors that you intend to use or is medium-resistant. This way you will have time to use and mix colors before they dry out (especially acrylic colors).
Last of all, your passion to paint and being right-brained always helps :-)