Why does Gin look blue in certain lights?
The effect in question here is when certain drinks glow in the presence of ultraviolet light. Gin is commonly mixed with tonic water and it’s the tonic water, not the gin, which will glow in the ultraviolet light. This is due to a component of the tonic water called quinine.
When ultraviolet light falls upon quinine, electrons in the quinine molecules absorb some of this energy and jump up to a higher energy level. The electron then loses energy giving it off in the form of photons as it jumps back down to a lower energy level. The photons given off are of lower energy than the ultraviolet light and correspond to blue light in the visible spectrum. This effect can be quite common in clubs or bars where ultraviolet lights are used.
Rik Sargent, Science Museum