"There are hidden harmonies or contrasts in colors which involuntarily combine to work together" Vincent Van Gogh
The color wheel is one of the most important tools for the artist. Start with the three primary colors: yellow, blue and red. These are primary because they aren't made by combining other colors. On the color wheel they are placed equidistant from one another. If it was the face of a clock, yellow is 12, red is 4, and blue is 8 o'clock. Simple.
So if you mix yellow and red you get beautiful luscious orange. Two o'clock on the old clock/color wheel. Mix red with blue and you get beautiful purple. 6 o'clock. What's left but mixing blue and yellow and making a gorgeous green, which is 10 o'clock.
Okay let's go back up to yellow. Going to the right around the wheel we have yellow, then orange. So what happens when you mix yellow and orange? That's right, yellow-orange! What about when you mix the next two colors, orange and red? You got it-it's red-orange! This is not rocket science. Pretty simple. Go around the color wheel mixing each pair of colors and you will arrive at the beautiful tertiary colors, as shown in the color wheel above.
Harmonious colors are close to each other on the color wheel. Yellow- green, green and yellow harmonize because they are similar to each other.