To elaborate, blue bubbles indicate that the message is being sent via iMessage, whereas green is SMS/MMS. iMessage is basically all the promises of RCS, realized. In the case of group messages, if just one person in the group is not on iPhone, the entire group has to fall back to SMS. The iPhone users get annoyed because they lose all the nifty messaging features that they’ve become accustomed to.
There are two obvious solutions here: Apple could embrace RCS, or Apple could open up iMessage to Android. So obviously this problem will never be solved.
Especially them.