Why has the color of my text msgs changed on only 1 of my contacts?

[Android Central Question]

I have a Samsung Galaxy. Earlier in the day the msgs were yellow & gray, later in the day they've changed. Only with this 1 person. Could they have changed it on their end?
I don't want to ask as it's a new client & I don't want them to think I'm not tech savy but I'd like to know what it means. Please help! Any info would be greatly appreciated. Thanks in advance!