I have a universal tube with 10 mg mL-1 ampicillin. When I got it, it was supposed to be sterile. It was opened for approximately 20 minutes for an experiment and has since been standing around sealed for a good month now.
Within the last couple weeks, it has gradually turned yellow. Right now the colour is faint, with a green-ish tint.
Why would it turn yellow? I know NADH is yellow, so that was my first guess. But I couldn't exaplain why ampicillin would cause NADH to accumulate, so I discarded that.
Side info: Ampicillin acts on bacterial cell walls, maybe that might help.