Well if protection from solar heat is the goal, it will be hard to beat the “chrome dome” or reflective parasol. Sometimes the ground reflects quite a bit of heat from below, like snow. Then I guess a shirt might out-perform a parasol.

Google also says to put glue on your pizza.

It seems it’s still an active debate and area of research, but the answer is more complex than wavelengths and emissivity. If you want to know whether black or white is cooler in the sun, it depends on: the breathability or knit, the amount of UV hitting the skin, the amount of skin contact with the fabric, wind speed, relative humidity, how the fabric wets and wicks moisture, and more. We could look at a black trash bag and say, well it’s transparent to IR, and it blocks the visible spectrum, therefore it’s a good shirt material to keep one cool. And obviously that would be wrong. In the same way it’s wrong to say: a white shirt feels less hot when you touch it, therefore it keeps the wearer cooler.

Body emits infrared radiation. Sun does too. They make foil-lined jackets to reflect this heat. White shirts do it too, as shown in the image.

Body puts off heat too. White reflects it back, black lets it escape.

This chart is very difficult to read for those with color deficiency - about 8% of men. Good and Fair are nearly the same, as well as Better and Superb. It would have been really easy to just use 5 shades of gray, or 5 colors that are distinguishable in greyscale.