Google conclusively found there was a statistically most pleasing shade of blue in their 41 Shades of Blue experiment. This type of testing can be and is being leveraged for profit. It's not too difficult to imagine Apple tuning iMessage vs. SMS colors to be perfectly calming/nauseating respectively.
It's also worth noting that the current version of iMessage uses contrast levels that don't follow Apple's own HIG (the early green bubbles did, but the shade of green has gotten lighter and lighter over time)
