I recently install Android 9. 0. since then I have noticed that the screens inside the phone app are a strange purple pastel color and the font doesn't seem to match anything else on the phone. I'm trying to understand whether the Android update caused this... Also, I noticed that Words With Friends forced an update on to my phone recently and it also uses the same colors. I don't know if it's a coincidence or if it somehow affected theme used by the phone app??
I just had something similar happen on my phone running Android 7.0 nougat, I think it might have been another system update that did it, maybe not Android 9. I did a cache partition wipe and it seemed to fix it. It's a very safe and easy procedure if you've never done it, but it varies by phone, so you'll need to look up how to do it for your phone.
Can you take screenshots and view them on another device? Do those colors come through in the screenshots?