Colour isn’t just whether or not light of any given wavelength is absorbed, it’s also about how much of that wavelength is absorbed. When light hits a thing, there’s a chance each photon will be absorbed or reflected, and that chance depends on the wavelength. If you have a bright red object for example, that might reflect 5% of green and blue wavelengths, but 90-95% of red wavelengths. There’s still a lot of light being reflected, but its mostly red light. A dark red object meanwhile might reflect the same 5% of blue and green light, but only 25% of red light. It’s still predominantly red, but there’s a lot less total light being reflected, so on the whole it looks darker. With grey, you’ve got the same proportion of each wavelength being reflected, but the amount is varying – a pale grey might reflect 80% of each colour, where a dark grey only reflects 20%.
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