What is the price of cool? About $31 a month, according to new research by Leonardo Bursztyn of the University of Chicago and co-authors. That is how much college students had to be paid to have their iPhone messages appear to others in a (lame) green rather than a (fashionable) blue bubble for four weeks. Introduced to indicate that a message has been sent by services other than Apple’s iMessage, the green bubble has become a marker for those either insufficiently wealthy to afford an iPhone or insufficiently aware of the stigma stemming from their preference for Android, another operating system. Better avoided, unless there is a reward.