This is an example of the watercolor effect. This effect was first demonstrated by Baingio Pinna in 1987.
Simply put The watercolor effect is perceived when a dark (e.g., purple) contour is flanked by a lighter chromatic contour (e.g., orange). Under these conditions, the lighter color will assimilate over the entire enclosed area.
What does that mean? Take a look at the below image. You see 9 distinct squares. The center of each square looks white but the outer area of each square has taken on a watercolor shade, pink, green, yellow, blue etc...
Created from samples
provided by Akiyoshi Kitaoka
The truth is that aside from the squiggly lines the only color here is white. The outer ring and the inner square are pure white.
Prove It: You can prove this by using the eye dropper tool in either of the following free graphics programs Paint.Net or Gimp or you can use the eye dropper tool in the free Colorzilla extension for Firefox browser. When you use these tools you will find that RGB value of everywhere on this image that is not a squiggly line is R:255 G:255 B:255 or #FFFFFF.
Print out this image and slide it into a plastic document protector. Now use an earasable marker and you have a reusable optical illusion Tic-Tac-Toe board.