Why

That's wrong. White holds the most. Black both adsorbs and radiates the most heat. Navy blue comes close, but since it's not perfect black, it's not a perfect radiator. That's how it is, and I don't know why.

The colour black is produced by the same radiation that enters the object as heat. It is one feature, not two. The ability to absorb heat usually produces the colour black when everything else is equal. If the radiation is reflected a little you get the colour navy blue. Reflect a lot, and you have white.

Thats a simplified version.