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.