This seems to be a test of the color accuracy of a display. I got 17/20 and the 3 I got wrong were the last 3. I did this on my iPad Pro. For the last ones, they all looked the same to me.
I wondered whether any display issues caused some to have no difference. I wonder whether it could somehow do a test to be sure your display is up to the task. It's annoying not knowing whether it's a visual limitation or hardware issue that causes wrong ones.
I just tried this on my desktop's monitor, which unlike my iPad Pro that was factory calibrated, has never been calibrated as far as I know. By round 11, the colors were nearly imperceptible, although I managed to answer 4 more correctly for a total of 14/20. Those additional 4 were likely more the result of luck than actual perception.