system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do red clothes appear gray / black underneath red lights?
gpt
If the room is only illuminated by red light, bright red pigment is indistinguishable from white pigment: they’re both reflecting nearly 100% of light falling on them (all of which is red).

If you have a dark red shirt that reflects 10% of red light, it will look the same as a grey shirt that reflects 10% of all light. The fact that the grey shirt is also capable of reflecting green and blue light is irrelevant because there’s no green or blue light around.