Gaze steadily at the cross, ALWAYS! Without moving your eyes, note the colors of the squares (red, green, blue, yellow). Every so often, different colors will flash up briefly. These colors are afterimages–not on the screen, but in your head!
The adapting plaid consists of a blue/yellow vertical grating, superimposed on a red/cyan horizontal grating. After adapting to this plaid, vertical black test lines make the afterimage look yellow/blue, while horizontal test lines make the after image look cyan/pink. Thus one and the same adapting pattern gives differently colored afterimages.
Conclusion: the visual system averages after image colors within but not across luminance test contours.
Similar results using checkerboards instead of stripes. Test squares look blue/yellow or cyan/pink depending on the position of the test contours.
Three-way pattern gives diminishing returns.