This one's absolutely diabolical.
Akiyoshi Kitaoka is a professor of psychology, specialising in visual perception, at Ritsumeikan University in Kyoto, Japan.
He's behind most of the optical illusions you've found online since 2002, the ones which make your brain really hurt.
This one is no different.
Here are two green and blue spirals among pink ones.
Nope, they're both actually this colour:
You can run it through photoshop if you want, they're both the same (RGB, R=0, G=255, B=150).
The reason this illusion works is a version of the brightness illusion called 'White's effect'.
Essentially the stark, contrast in colours makes it hard for our brain to process each colour individually, when positioned close to each other - as a result we see the same colour as slightly different.