The primary colours (red, blue, and yellow) form the basis of the colour wheel. Secondary colours (green, orange, and purple) are the colours formed by mixing the primary colours. Tertiary colours (yellow-orange, red-orange, red-purple, etc) are the different hues formed by mixing a secondary colour with a primary colour.
Complementary colours appear opposite each other on the colour wheel. Although they are the strongest contrasting colours, they go together well. The colours that appear next to each other on the wheel (analogous colours) are also considered harmonious. Finally, colours that appear together in nature generally go well together regardless of where they appear on the colour wheel.