T.S.
As an artist myself, I can say that sometimes we get very involved and energetic when making something. Now as an adult, I can focus more and really get into what I'm doing, as a small child, he doesn't know how to really focus the way an adult can. If you are worried about him putting lots of colors together to make black, try two colors that will mix to make a new color. He will learn color combinations that way. You could probably find a color wheel online and see what colors mix and match. Please don't criticize him. Instead of reasoning with him to draw/paint the way you think he should, when he is getting 'into it', try asking him what he's doing, 'what are you trying to make?', 'what were you thinking about when you made this'... he's expressing himself (as all artists do) so maybe trying to get him to 'explain' his art creation will help him look at what he's making in a new direction and it will help you understand what he is feeling when he gets all crazy and artsy. Let him create the way he wants and be happy if he is pleased with it. The worst would be if he was making a painty mess and getting frustrated by it (that's not healthy)... but he's not. So let it go and let him have his fun.