Incoming I Messages appear in a GRAY bubble

My wife has an older IPhone 6S. She's had it for several years and it still works OK. About two days ago incoming IMessages from her phone appear on other IPhones in a GRAY (instead of the familiar BLUE) bubble. All the phones have a good WiFi connection, and I Messages is turned on. The only difference that I can see is the color of the text bubble. I know about BLUE, and i know about GREEN, but I don't know about GRAY. Is this something new?