Ok ...when you have a message up ..hit the 3 dots...several things will come up...delete etc.
Hit the CUSTOMIZE.....this will add color to your messages ..in or out.
I totally agree this ugly real sucks I didn't realize you couldn't change the color until I read this thread because I do remember changing it many times on my old Samsung phones. Thank you! Now I don't have to waste 2 more hours trying to figure out how to do something that isn't possible..😑