Why am I getting a red tint in the lock screen in Oreo 8.1/Nexus 5X?
Google support says that the tint is based on the colour of wall paper and there is no way turn it off. That's for Pixel phones and perhaps same applies for yours

Like you said the color tints are based on your wallpaper. I've found it typically does a nice job with this sorry you don't like it. There is not a way to turn this off.

Edit The colour tint applies to lockscreen too going by Android Police blog

Lock screen
It looks like the lock screen does this as well. However, the effect is even more subtle there.