The part that I don't think you understand about the paint is it keeps the unit stable when the fan is not running. That is when people are getting errors and warmer readings than their surroundings. When the fan is on, that keeps the air mixed, and you get mainly accurate temps and wet bulbs. However, when the sun is rising and setting, the dark color absorbs the sunshine rays, and it fouls up the sensor, and you run up to 10 degrees high. It's been proven over and over and over and over.
I understand how fan aspiration works. That's why it's important to keep the aspirating fan running properly on the 5n1 until the heat load goes down. That has been "proven over and over and over and over".
My fan is working fine, and that's why I don't have problems. I also don't let mold grow on my sensor. I clean it occasionally....blow the dust off the solar panel, check alignment to true (not magnetic) north, lubricate a bit with silicone spray, remove spiders, etc.... but you didn't want anyone talking about those things, remember?
If "painting the bottom" was a silver-bullet solution, don't you think someone would have stumbled on that a long time ago? You're not exactly the first person to have that idea. Others have also tried things like white tape, white plastic skirts, and other contraptions. They haven't proven themselves effective.
However, what always works is keeping the aspiration fan running properly, even if that means adding extra panels or other auxiliary power.
To sum up, if your fan isn't running, maybe it should be.