好菇凉比白纸还白坏骚年比乌鸦还黑
[hăo gū liáng bĭ bái zhĭ hái bái huài sāo nián bĭ wū yā hái hēi]
A creative play on Chinese characters ('菇凉' is used as homophones with girls), meaning 'good girls are pure/innocent (whiter than white paper), while naughty boys/wicked youngsters are blacker than crows (indicating evil). This highlights extreme moral contrasts.