With watercolours, what is the diefference between artist and student quality paints and how many colours can be successfully mixed before you get a sludgy effect?

Ray Campbell Smith replies: Artist quality pigments are more finely ground and sometimes contain more expensive ingredients. I think they are worth the extra cost for their strength and clarity. I would never mix more than three colours together - preferably only two. Remember that many tube and pan colours are tertiaries and already contain all three primaries.