When the Newton’s disc rotates, all the colors actually fade to white - and that is a strange occurrence that you don’t expect out of painted patches of color! So what really is happening here?
To understand this phenomenon, we have to first see the seven colors that make up the segments of the Newton’s disc: red, orange, yellow, green, blue, indigo and violet. These are the same seven colors we see in a rainbow.
Now, when this is rotated fast, the colors undergo what we call additive mixing, wherein the colors reflected off of each segment enters our eyes collectively (red light reflects from the red segment, blue light from the blue segment, and so on). Adding to this is the persistence of vision, wherein the eye cannot distinguish details when the segments move fast, so we see them “blur” or “blend”. This ultimately causes all seven colors of reflected light to “mix” on our retinas.