The colour wheel arranges different colour hues around a circle. Three primary colours are placed at equally spaced points around the circle. When two primary colours are mixed, they create a secondary colour. Mixing a primary and secondary colours creates tertiary colours.
There are two important colour models, which have different primary colours and therefore different colour wheels:
Additive colour is based on how coloured light behaves and combines to form different colours. It is important in digital media or stage lighting. Primary additive colours will combine to form white.
The primary colours in the additive colour model are red, green and blue (RGB). Secondary colours are cyan, magenta and yellow.
Subtractive colour is based on how coloured pigments, such as paint behave. Artists typically use a traditional subtractive colour model, such as Itten’s colour wheel, where the subtractive primary colours are red, yellow and blue. Primary subtractive colours will combine to form black.
Printers use a more accurate CMYK colour model where the primary colours are cyan, magenta and yellow.
Complimentary colours are those positioned opposite each other in the colour wheel (which colour model??) These colours effectively cancel each other out so are said to have the most contrast.
My own experiments with colour mixing using watercolour paints (subtractive colour). Alchemy!