When I was a kid I had a black light...

Pure gray is function of luminance, not of colour.  In the colour gamut visible to the human eye, chroma decreases with luminance because the colour sensing cone cells are not as sensitive as the luminance sensing rod cell, so colour goes away as the light level decreases.

I suppose you could have a brown light, though.  Try mixing red with some green.  Makes me think of an old Zappa song: "Ronnie saves his noomies on the window in his room--a marvel to be seen--dysentery green..." :ohmy:

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