Quite a brilliant bit of UX design on Apple's part to put SMS/MMS and iMessages in the same exact app and colour them differently to form an instinctual association in the user's mind that 'green bubble bad, blue bubble good'.
It’s worth noting that all messaging used to be green back in the old iOS days. When iMessage was added they made them blue so you knew you were using the new snazzy feature. So to those saying apple picked an intentionally derogatory color are simply misinformed.
