There's an interesting guide on how to make radiative cooling paint from common ingredients complete with an explanation why it works:

https://www.youtube.com/watch?v=KDRnEm-B3AI

The catch is that such paint is not particularly robust, much less weatherproof.

Beats yogurt I guess, which tends to smell as it turns in sunlight.

I have roof windows and was wondering how much I could gain from panels painted like that, but most likely as much as I would using the next best thing - a window sunshade, which I'm using already.