Spiral illusion

The below image has an optical illusion. Can you spot it?

Blue/Green Illusion 2

Believe it or not – and I didn’t – the blue and the green are the same colour. I had to zoom in in Photoshop to prove it to myself. There’s a close-up here.

Looking again at the full-size image, I can just pick up a green tinge at the edge of the upper-left blue, but that’s all. Impressive. I’d be interested to see a writeup of it at Illusion Sciences. Via Richard Wiseman.