Color Contrast Optical Illusion

Below is another excellent example of White's illusion.

This is an interesting effect, below there is only one shade of purple. Let me be clear both shades of purple have an RGB value of R 198 G 173 B 231.

Although the purple on top seems to be darker than the purple below, they are actually identical.

As you can see, the purple triangles on top are mostly surrounded by blue, while the purple triangles below are surrounded by white. The color triangle's surroundings are changing their perceived color.