When we are studying the color wheel and then throughout the year, I will line up the kids by asking them questions like this. Red + ? = pink, red + yellow = ? Purple-red=? We may do this when we have a few extra minutes to line up. This would be 3rd or 4th grade.
> How about X + Red=Orange
> Y + X=Green
> solve for Y
> then there's always the word problems
> Sally is dressed from head to toe in X, but doesn't want to clash with Jane who is wearing green which is X's complement. "Together we will look like a Christmas tree" she wailed.
> San D