Vision can be affected by drugs and alcohol making especially difficult to distinguish.

Explanation:

Drugs and alcohol consumption affects the iris functions, specifically, pupils.

Pupils is the center of the iris, its function is to let pass light inside the eye. On the other hand, colors are derived from light, it's the source of colors. So, if drugs and alcohol make pupils contracts more than normal, you won't normally perceived colors as such. If your pupils are too dilated, it's gonna enter too much light, if your pupils are too contracted, it's gonna enter not enough light.