Text bubble randomly changing color on Galaxy S6's stock Messaging app on Android 6.0
After a lot of research I haven't (and from the comments, it looks like neither have other users) been able to find any resources on the internet regarding this curious phenomenon. 

I am, therefore, forced to accept this as some sort of Easter egg from the Samsung developers' side (although, in my opinion, Easter eggs should at least look nice.).

This also means that you don't really get an option to 'opt-out'. If it really bothers you, you can Contact Samsung (although their 'Live Chat' feature seems badly designed since I couldn't get a representative to talk to me even after 30 minutes) or use another messaging app.