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 the

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.