Interesting. I only got 15/20, and previously considered myself "above average" at colour distinction tests but based on other replies that's not an especially good score. I'll try again, going more carefully.
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The first time I kept my eyes fixed in the same place roughly in the middle which clearly wasn't a good idea. On the second attempt I glanced between each circle in turn, trying to discern the difference over two points in time rather than two points in space.