Baffled by Gray Outgoing iMessage Dialogue Bubble

For the very first time since I got my first iPhone in 2007, an outgoing iMessage that I sent to a frequent iMessage recipient did not show up in a blue or green dialogue bubble, and I have a clear understanding of the reasons behind these colors and when and under what circumstances to expect them. The gray bubble, however, being its first time to appear in 16 years, has me truly baffled. Virtually all the responses that I have read to others who have posted this question are simple regurgitations about the distinction between blue and green bubbles, while in no way actually answering or even addressing the question that I'm posing. I'm hoping there's a soul out there who will, without explaining the blue and green schema again, be able to offer an explanation as to why for the first time ever when I sent my friend an iMessage that it appeared gray. Anyone? I'm also willing to admit that the answer might already be out there somewhere and I missed it, so if somebody could perhaps direct me to that if it exists, that would be really appreciated!