The color of the bubble isn't the problem. iMessage uses a green bubble as a indicator that your iMessages are being sent as SMS, which is an old, terrible standard.

Imagine if 90% of people used WhatsApp/Discord but 10% of people are using the default messaging app on their phone. Those 10% of people are causing you to have shitty video/picture messaging. No typing indicators. No read receipts. No delivery confirmation. No in-line replies. Now imagine if you're in a group chat with several people, but that ONE person causes ALL of you to have those limitations. The group would be pissed off at that one person. That's blue bubble/green bubble bullying.