Just don't give your kid any colors that, when mixed, turn to muddy brown! That means that instead of offering the whole pallet of colors, you should only give two primary colors. When the colors mix, you'll get a third color.
You remember your color wheel, don't you? If not, here's a refresher:
Your primary colors are Red, Yellow and Blue. So you can give the following combinations with the resulting third color:
Red + Yellow = Orange
Yellow + Blue = Green
Blue + Red = Purple
In Milo's painting above, I gave him blue, yellow and green paint (I went ahead and threw an already-mixed green for him because, why not?). To add in more variety of color, I offered him glitter glue in magenta, orange and blue. Since the glitter glue won't change color when it is mixed with the paint, he was able to get lots of colors without the whole thing turning into a big brown blob. Not that brown can't be lovely, but sometimes we want beautiful, vibrant colors! Especially in a painting of the ocean with islands and pirate ships and treasure such as is the painting above (or so I am told).
Miranda R. Mueller Original art for children and their rooms ❀ DIRT Cosmetics Made from earth