都是白开水别装优乐美
[dōu shì bái kāi shuĭ bié zhuāng yōu lè mĕi]
Literally 'All are plain water, don't pretend to be Ureshii Lemon Tea'. This implies not putting on airs or trying to seem like something one is not, encouraging authenticity rather than artificial appearances of glamour or superiority.