purple text message bubble Iphone X iOS 11

A very helpful agent (Vijay) at Apple Support solved this mystery.

If you go to *** Settings > General > Accessibility > Display Accommodations > Invert Colours ***, the "Invert Colors" has three settings, "off", "Classic Invert", and "Smart Invert". My iPhone was using the third setting. I could not change the text bubble colors, but this algorithm could and did. I finally managed to remember that I had chosen this setting by following advice on tricks to increase battery life. I have turned this off in order to avoid the purple haze.

No The bubble is definitely purple

We have two phones IPhone 6 and iPhone 10, both running iOS 11.4.1. The funny colors do not appear on the iPhone 6, so something has been reset on the iPhone 10. I don't know how, since I didn't do it. So far, everything I see says that these colors cannot be reset. I will as Apple Support about all this.