Why did my screen change color my pictures look like negatives now

You might have high contrast mode enabled. With all the different flavors of Android around, I can't help you specifically but you might want to look at Settings and see if High Contrast is on in Accessibility or Display. You haven't provided much information about what caused the issue or if you've tried to turn it off and back on again. Speaking of, try turning it off and back on again, along with the things I mentioned earlier.

Just go to settings, and select “Accessibility” You’ll see a choice for vision. Select vision, then turn off “Inversion.” That should fix it for you. (Note: this is how it works on my LG phone. I imagine it should work in a similar fashion for your phone, too.)

Depends on your phone and/or o/s. In

My phone is a LG K20+. I noticed this this morning suddenly. I tried restarting the phone, but it didn't work.

Settings, accessibility, color inversion, turn off, make sure it's off,