Colour illusion

See that grey image below? Click on it, and stare at the dot inside the 'S' for 30 seconds or so.

Then — without moving your eyes — click on the image again. The image should appear to be in colour until you move your eyes again. Magic!

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The original photo is here. Thanks to John Sadowski for his Colour Illusion Tutorial (for Photoshop, but it still works great with GIMP). Check out his original Spanish Castle illusion.