I also got a zero, and I just barely noticed what you mean--I think it had to do with the white cursor being over the block. Probably similar to this illusion:

I also got a zero, and I just barely noticed what you mean--I think it had to do with the white cursor being over the block. Probably similar to this illusion: https://en.wikipedia.org/wiki/Checker_shadow_illusion https://en.wikipedia.org/wiki/Checker_shadow_illusion I noticed I had to spend a lot of time in the bottom two rows near the ends--in the purples, for instance, it was near-impossible for me to tell the last three tiles apart.