Aftertone
A gray patch on a bright field looks darker than the same gray on a dim field. That is simultaneous contrast. Matching what you remember often means compensating for the surround — and overshooting the true lightness.
Drag the slider and fine-tune after you let go. You may view the sample once more; your score is not capped for that.
The scoring formula for Aftertone is published on How to play, and the constants there are still initial values.