I use primary colours when painting with clients because it's exciting to see the colour changes with mixing. You can mix every colour in the spectrum with the primary colours and white. Colour changes are often unexpected for clients and it is one of the most engaging qualities of the media.
'Sam', a 14 year old and non verbal client of mine was particularly delighted in mixing colours, often layering numerous colours over each other using broad strokes at various angles. At one point, on seeing a particularly vibrant colour and contrast emerge he said "Yes' quietly but clearly to himself as he finished the stroke and lifted the brush from the paper.