The green predates the existence of iMessage. That green is the color that was used from the beginning of the iPhone in 2007; iMessage was only added in 2011. iMessage was given a different color than the preexisting one.
Trying to claim that Apple picked an "ugly color to make you feel uncomfortable" is simply false.
The original green bubbles used a different shade but more importantly they used black text. iOS 7 switched to white text and a shade of green that means the contrast for SMS failed Apple's own guidelines. Even for people with decent eye sight it can cause additional eye strain and be perceived as ugly even if the green on its own would be fine.
To be fair the blue they choose is also not ideal, but it is a lot better than the green.
Trying to claim that Apple picked an "ugly color to make you feel uncomfortable" is simply false.