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.
SMS is “texting”. The same shit that has been around since cell phones.
Think of iMessage, Discord, WhatsApp, etc. as RCS or “messaging”
When Android and iOS text each other, it all sends as SMS. That means no read/delivery receipts, low quality picture/video messaging, no typing indicators, no encryption, etc.