I think it's even simpler than that. Any black body that is colder than its surroundings heats up. Any black body warmer than its surroundings will cool down.
This thermos would only keep your coffee hot if you dropped it into a furnace.
You could make a colored flask which absorbs visible light (from the sun) but reflects thermal wavelengths (from the drink). But due to infrared leakage and thermal conduction, I don't know if it would keep drinks above 100 degrees F for anywhere near as long as a standard vacuum flask.
The neat thing about wearing white in the desert is that it keeps you cool during the day and warm at night. Wearing black does the opposite.
A perfect black surface on a spacecraft can do more than charcoal black paint, but only by a matter of degrees. If memory serves its less than a 2x multiplier between off the shelf and exotic materials. But when ounces cost thousands of dollars, you’re going to go exotic.
This thermos would only keep your coffee hot if you dropped it into a furnace.