The author is just overthinking things here. If anything, green is the color used to indicate “go”, a positive sign that has been etched into our minds. The reason is very clear as already mentioned “it doesn’t support messages”. Why isn’t that explanation enough?

EDIT: to clarify here, the author does explain his point about the color and the contrast of the text. He may have a point there but IMO, the main issue was that it’s not messages. The thing I should have added is that back when it was implemented people had to pay real money per message to send as a text message. That means everyone in the group had to feel uncomfortable sending frivolous messages when it costs someone money. I think that is the real reason for the bifurcation and not the color choice.