The color of the bubble is only important because it helps iPhone users know who not to add to group chats, since the presence of a non-imessage user in an iMessage group chat downgrades the entire chat to grainy photos, no reactions/ read receipts, voice memos, typing indicators, etc. I don't blame them at all, many of them don't use any third party messaging apps because iMessage is built in and gives them everything that other chat apps have, with the benefit that they don't have to convince anybody to install it because all their iPhone owning friends have it preinstalled.