People should try to keep in mind the original purpose of bubble colors.
SMS is green, because it (used to) cost money. Green is the color of money. Carrier plans would include a small number of texts with pay-per-message billing after that number was exceeded. It was a thing. It was horrible.
iMessage is blue, because it is freeeee. Any data plan - and postpaid data plans in the US at the time universally included unlimited data - can easily absorb the amount of data used by text only messaging. Apple wanted to make it clear that these messages are different, even though you use the same app to send them. These messages don't cost you money.
These days, the bubbles work for Apple in encouraging young people to harass their friends into getting iPhones, so Apple will leave things the way they are.