Color from flanking contours
from Michael’s Visual Phenomena & Optical Illusions
Above are 4 quadrants, covered with trellis patterns, and colored backgrounds. Look at the one with the yellow background: what about the color of the trellis? Looking blueish to me. Top left they look reddish to me.
As you probably already guessed, there is an illusion here: All trellis lines are black (or, rather a very dark gray), no color to them at all! And the hue shift is caused by very thin white contour lines, which are flanking them. Try unchecking “Contour”: all trellis lines are without color now.
There are lots of parameters you can explore: line thickness, contour width, trellis scale, all colors…
The color change clearly goes into direction of opponent colors, so it looks like “simultaneous color contrast” here. Surprising is that the thin white lines are the cause. All the more, because “thin” or “fine” is normally associated with color assimilation, which goes into quite another direction. Kanematsu & Koida (see below), who first reported the present effect, consider with utmost care many possible explanations, e.g., chromatic aberration (and convincingly reject it). But so far this is ill understood. And that’s a really complicated paper…
Kanematsu T, Koida K (2020) Large enhancement of simultaneous color contrast by white flanking contours. Sci Rep 10:20136. [No paywall!]