红内裤还不得大姨妈红
[hóng nèi kù hái bù dé dà yí mā hóng]
It literally refers to menstruation humorously and indirectly, meaning something like 'The red underwear does not equal menstrual blood red'. This indicates a casual, down-to-earth attitude about feminine hygiene issues. However, this term could be considered slang and not suitable for all situations.