Most often in two contexts: (1) a sickly face —
脸色青黄. (2) The set phrase
青黄不接 (qīnghuáng bùjiē) 'the green (new crop) and yellow (old harvest) don't meet' — meaning a hand-to-mouth gap between old supplies running out and the new harvest not yet ripe. By extension: any temporary shortage / generation gap.