I have noticed that most objects such as the red dye in a plastic stool look black under blue or green light.
From what I understand about direct band gap semiconductors, the dominant recombination mechanism is radiative recombination. Therefore If the dye was a direct band gap semiconductor I should expect it to still appear red under blue or green light as the electron in the red dye should absorb the light, thermalize and radiatively recombine releasing red light that's equal to the band gap in its energy. Therefore if it was a direct band gap I would expect it to fluoresce red in blue or green light.
The fact that it appears black implies there is another recombination mechanism such as indirect recombination that would cause the blue or green light that is absorbed to be entirely lost due to heat rather than by radiative emission.
Based on this I would assume that all indirect semiconductors would appear black, but for direct semiconductors, they should fluoresce.
I also understand that materials can have both indirect and direct band gaps. But for wide direct band gap semiconductors that have an indirect band gap that far offsets the direct band gap in energy should fluoresce under blue or green light.
Is my understanding correct or is there something I'm missing? Many thanks.