Hueseam
A wide color bar hides a faint vertical seam. Slide the marker to the edge you can just make out. 4 rounds.
About Hueseam — find the hidden seam
Hueseam trains edge detection — finding the faint boundary where one shade gives way to another inside a wide color bar. Because the two halves are so close, the seam almost dissolves, and surrounding color pulls your judgement around through simultaneous contrast. Locating soft edges like this is what lets you catch banding, spot where a mask feathers wrong, or see a subtle join in a printed gradient.
A broad bar hides a barely visible vertical seam between two near-identical shades. Slide the marker to the exact edge you can just make out; four rounds make the seam ever fainter.
Your surroundings bias the boundary through simultaneous contrast. Read that guide, or browse all our Learn about color articles on the science of color perception.