How do I change my iPhone texts back to blue?

Edit
My iPhone 7 has this group chat on iMessage with many of my friends from acting class. The text usually sends through in a blue color, however, sometimes it turns green and although I text in green on that chat, the text creates other separate chats for my friends. I can't seem to find out how to change it back so it won't create other chats and so that I can contact my friends on the same chat.

The chat are green because someone is texting from an area with bad reception or an android phone (this means it is using data to send the text messege) there isn't really a way to change it back.

If someone in the group is on an Android phone or if either you or they are in a bad signal area, it'll be green. And you can't respond to only one person in the group from a group message. You would have to send a separate text only to that person.