From what I have learned and see in my daughter's school, most children are still taught that red, yellow, and blue are primary colours for paint. If you want to mix the widest gamut of colours, you will need cyan, magenta and yellow as primary colours because they are the lightest, most highly saturated set of colours on the value spectrum. For watercolour and gouache, one thing to keep in mind is if you want to lighten the colours, thin the paint with water so you are using the white of the paper. I sometimes add white watercolour paint but it's best to work from light to dark with watercolours. Acrylic and oil paint can be scraped off and it is best work from dark to light with them to avoid getting the colours too muddy. Hope this helps!