Why does newton's colour disc appear white on rotating ?

Because when white light splits it consist of seven coloures VIBGYOR.tHEREFORE WHEN THIS 7 coloures rotate they form white light.

it shows that white colour is made of seven colours

When we pass sunlight through a prism,it appears to be the colour of rainbow. We know that the earth is rotating at a high speed so sunlight appears white.Similarly when the newton's disc ia rotating,it appears white.