Source: Tom Cook's answer to Partying: Why are red Solo cups so popular? - Quora
Stashed in: Best PandaWhale Posts, Alcohol!, Quora!, Colors!, BRO!, FALSE., Alcohol, snopes!
Seems about right:
I love that not even the people at Solo know according to Slate.
Red Psychology says that red is more attractive.
Plus, red cups are more fun. You won't see people marking up a blue cup thusly:
Snopes says the lines corresponding to measurement is just a coincidence:
Red Solo Cup: