There's no need to stare at a single point on a grid.
If you look at the black-and-white grid again, you should notice a green haze around the horizontal lines, and a magenta haze around the vertical lines.
Further, it it is a phenomenon: the colour binds to the orientation of the grating: Green was horizontal, and the horizontal stripes show the faint magenta aftereffect.
The mechanisms of the Mc Collough Effect are not understood in spite of 50 years of research.
The effect typically lasts for hours, or even overnight.