One thing you *could* do (although I'm aware that it'd be a little out of keeping with the rest of your grayscale icons) would be to make the lemon, and only the lemon, yellow. That would also make it stand out from the rest of the icon.

I might be the only person who has trouble making the lemon connection, though, because I'm sort of notoriously dense at picking up the finer points of visual metaphors. (I'd actually thought it was a squeeze bottle of something.)