Blue, as in DEPRESSED.
The Big Bang Theory Season 4, Episode 22
Are you DEPRESSED because you're
alone and no one loves you?
The Big Bang Theory Season 4, Episode 5
...green is jealous
and blue is DEPRESSED.
The Big Bang Theory Season 4, Episode 22
It's Christmastime and
Jimmy Stewart's really DEPRESSED
The Big Bang Theory Season 7, Episode 11
...because I'm fun,
I'm not clinically DEPRESSED at all.
The Big Bang Theory Season 6, Episode 5