Color Theory is essential when it comes to painting. This is a traditional color wheel showing the pure color (also referred to as hue). You will see that each color is then tinted with the addition of white, toned with the addition of grey and shaded with the addition of black. This is the most simple way to control value with color, note that an even greater range of tones is possible by mixing more ore less white, grey or black accordingly.
This exercise is called a color identity test. Here the student is working with paints straight from the tube, unmixed. Each color is applied as a thick "chip" of paint and then thinned with linseed oil in the box below to see how the color acts as a transparent glaze and with the addition of white in the box below that. The true nature of a color is difficult to read in its pure unmixed form. Adding white or thinning the color allows us to see the temperature and "feel" of the color much easier. The pure color unmixed is said to be fully saturated. Chroma is another word that describes how bright or intense a color is.
This student color chart shows color tone gradations matching a grey scale, color wheels, and neutrals made through the use of compliments. Each "test" shows a different effect and range of tones and color saturations. Note the bottom color wheel is using the mixture of compliments to achieve neutralized tones.
This scale demonstrates the use of complimentary colors to achieve darker values. At some point, the chips will be so balanced that the color is neither one nor the other. This chip is what we would classically call mud, a color that's over-mixed with it's compliment or is too neutral to have a color identity.
This student mixing chart shows neutral gradations of various pairs of colors (generally complimentary colors).
This final student color wheel chart shows different triads. Color wheels can be made of as many colors there are tube pigments. Which red, which yellow, which blue should be used? The reality for painters is that we are dependent on pigments, not just theories of colors. Experimenting with many color wheels and many primary triads is important and closer to the truth for painters. Note the earth palette color wheel of yellow ochre, ivory black (blue) and venetian red is a much less saturated version of a traditional color wheel.
Additional information here
*Al Gury's Color For Painters is an excellent reference and resource I found to be extremely useful and have referenced here, copies can be found on amazon for as little as $8.