Don't know if this is new, but I noticed today that my replies to a text message bubbles are a different colour from yesterday..? Will be interesting to see if it changes again tomorrow.? If so a nice little feature that allows you to see quickly conversations over a certain day.
I don't think the color of those messages should change by itself
Like @ARamsay mentioned it might have been due to RCS being activated for that particular conversation.
Dark mode and/or a new theme might have also affected that.
If none of those seem like it could happen for your device, then try submitting an error report in Beta Feedback using the Samsung Members app!
Are you sure its not the difference between an RCS message and an SMS? (It works a bit like iMessage with different coloured bubbles)
Yes you are correct on this as I only found out when a RCS msg failed to send, shame it didn't give me a popup asking if wanting to send as SMS instead..! I had to go into settings to change it to SMS mode.