Looking at the colourwheel above, you'll be able to see the three primary colours (yellow, blue and red) They are called primary as they can not be created from other colours. All other colours come from them.
If you mix equal measures of red and yellow you get orange.
If you use more red than yellow, your orange will be reddy orange.
If you use more yellow than red the orange will be more yellowy orange or golden.
If you mix equal quantities of blue and yellow you get green.
If you use more blue than yellow your green will be teal toned
If you use more yellow than blue, the green will become more citrusy lime
If you mix equal measures of red and blue you get purple.
If you use more blue then red you get rich purple.
If you mix more red than blue you get magenta.
By adding white to any colour you get lighter pastel tones as in the wheel below the half wheel)
By adding more white you become neutral (barely any colour tone) as in the half wheel below.
Black through to grey are not colours. They are tones and are neutral.
Colour can be used in any way depending upon the look you want, but the general rule of thumb are:
Keep your colour choices with in families. Families denote the depth of colour.
You can also take colour from section of the wheel and they'll sit beautifully together. my personal favourite is to slice blue, purple and green together. (again the celtic women offer a look at this option because the palate is limited, they can combine depths and tones)