It looks like in iMessage for business, the phone’s user’s outbound messages show as dark grey (as opposed to normal iMessage and SMS/RCS which show outbound messages as blue and green respectively). I assume this is supposed to communicate that you’re talking to a different sort of entity, not a normal person on a phone.

Personally I don’t see why you’d care. My business isn’t trying to pretend to be a normal person using a phone, so why would it matter?

https://www.apple.com/ios/business-chat/

I’ve never seen a gray bubble and have received incoming messages from all different types of accounts

Then there’s a misunderstanding here. I have an iPhone. When I open my Messages app and view a conversation with someone with an iPhone, my outbound messages are blue and their messages to me are gray. When I open a conversation with a non-iPhone user, my outbound messages are green and their messages to me are still gray. Are you sure you’ve never received a incoming gray message? Because that doesn’t seem possible, unless you’re talking about something other than what I and the person you responded to are talking about.

As far as I can tell, the OP’s insistence that their service won’t send gray messages seems entirely disconnected from reality.

yes, what I originally meant was that Apple iMessage for Business has gray bubbles for the messages that you're sending (or the ones that would normally be blue)

Incorrect.