青檬不比柠檬酸
[qīng méng bù bĭ níng méng suān]
'Green Lemon Isn't Sour Like Lemon', metaphorically suggesting that sometimes, despite outward appearances of resemblance (in color, form), things can differ in essence. It plays with words on appearance not necessarily meeting actuality (lemons and limes both being citric fruits, are naturally tart or sour but might be perceived differently).