On the right you see a horizontal colored grid, where every second the gridline colors are interchanged. Furthermore, there are two vertical stripes which look greenish and blue with the default settings.
It seems unbelievable: there are only 3 different colors! The greenish and blue are actually made up by identical green segments, only the intervening grid lines are different.
What to do
Convince yourself that really only the background gridlines change:
Use the pop-up menu with preset color schemes or set your own color combinations with the 3 color pickers.
This illusion shows how much the perception of color is influenced by the neighborhood. The keyword here is assimilation.
Closely related is the Munker-White illusion which shows strong luminance shifts.
Munker H (1970) Farbige Gitter, Abbildung auf der Netzhaut und übertragungstheoretische Beschreibung der Farbwahrnehmung [Chromatic grids, projection to the retina, and translation theory-based description of the color perception]. München: Habilitationsschrift [some pages from that work on Kitaoka’s beautiful site]
White M (2010) The Early History of White’s Illusion. Colour: Design & Creativity 5:7,1–7
Monnier & Shevell SK (2003) Large shifts in color appearance from patterned chromatic backgrounds. Nature Neuroscience 6:801–802
Akiyoshi Kitaoka has nice spiral versions of this