Why did you see the color white (or possibly grey) when you looked at the spinning Newton color wheel?
When the color wheel is spinning fast enough, your eyes can no longer see each individual color. Instead your eyes just see a blur of the combined colors. And when all the colors of light mix together the result is white.
A good way to think about this is with a prism. When white light hits a prism the white light splits into all the colors of the rainbow. This happens because white light is actually made of all the colors of light.
Each color has a different wavelength. You eyes can detect these different wavelengths and that’s how you can tell the differences between the colors. But when all these wavelengths enter your eyes at once, you see white.
And that’s what happens with the color wheel.