White light is the mixture of all colors.
A prism breaks light up into its component parts, violet, red, orange, yellow, green, and blue.
A diamond functions as a box of prisms.
If you were in a dark room with black floors, walls and ceiling wearing black clothing and there was one tiny halogen light in the ceiling you'd see vivid rainbows.
Add a second halogen light and you will see some more rainbows.
Another and you will see even more rainbows.
But at some point if you add too many light sources the light from one halogen will start mixing, blending and combining with the light from another.
The result of this mixing is you will no longer see the separated colors.
This is because it works both ways . . . white light can be broken up into colors, but colored lights can also be mixed to get white light.
Take three flashlights with colored glass over the front, one red, one green, and one blue.
Shine them all on one place and you will see white.
This is what's happening on a cloudy day inside your diamond.
We usually think of the sun as the only light source in the sky but it is not.
On a cloudy day. . . the light source is huge, the whole sky.
You can think of it as trillions and trillions of halogens forming a continuous dome over your head.
Zillions of light rays are all entering your diamond from all angles.
The white light is still being broken up by the diamond but the zillions of other light rays entering at zillions of slightly different angles blends with the others and mixes or cancels the color so instead of seeing the colors you see the mixed colors, or white light.
If you see rainbows or colors coming from your diamond, even on a cloudy day, something in the environment has to be brighter than the rest of the environment.