These are RGB color wheels, with red, blue, and green as the primaries rather than red, yellow, and blue. RGB are additive primaries, which is what TVs, phone screens, and computer screens all use.

In the additive model, if you add red and green together, you get yellow. But if you add the opposite color, cyan, you get white.

In the subtractive model, where CMY (or cyan, magenta, and yellow) are the primaries, adding yellow and cyan makes green, but adding opposites like yellow and blue makes gray, or black.

I made the color wheels above to be used for painting, so I’d be using them with a subtractive mindset, and that’s why the center is gray, rather than white.