[rsfinn]

At the time the green/blue distinction appeared in Messages, it was widely reported that the green color for SMS messages represented the fact that most cellular plans in the US at the time charged a fee for each SMS message sent, or had a higher monthly rate for "unlimited" texts. It would seem that very few people remember the days before "unlimited data/text/minutes".

(US currency is typically associated with the color green in a way that probably doesn't apply elsewhere in the world. Also, the actual color of green on US paper currency is quite different from the green bubbles... but I digress.)