Meanwhile, limewash (https://en.wikipedia.org/wiki/Whitewash) is cheap, simple, environmentally friendly, and extremely effective - and has been used for millennia all around the Mediterranean.
Anecdote: A friend of mine chalked his roof. It got his interior temperatures down by almost 4C. This was after shading all windows and doing the obvious things.
Cool! More reason to buy paint made of this stuff if it ever goes to market.
My concern would be degradation of efficiency over time as the roof gets dirty. If we're talking about a few percentage points of efficiency, cleanliness will make a big difference. I doubt the paint is robust against frequent cleaning.
This sounds like it would be much better than cheaper alternatives (like whitewash mentioned by others) for one summer, but just one summer.