iMessages changing color

I’m finding my iMessages from a contact previous history turned green when I know for sure they were blue before. Why has it changed? Even though when I text current messages now they are blue. So for this contact my past conversations turn green and anything from present on our blue. Why has that happened? I’m trying to determine if the messages went through another messaging service in between the contacts phone and my phone. Could that be the reason why I’m getting this new change in color?