Two color wheels, one is the opposite color of the other, and is behind the other, but the other is half transparent, this makes it look grey, but the square is inbetween, so it covers up the one that's the opposite of the other.
Thanks to @MartinBraendli2 for his interest project https://scratch.mit.edu/projects/175039437/ I learned this from his project. I suspect I can improve this more.
