Can't make out the difference between the purple coloured tiles. Color-blindness?
If you're actually wondering, here's a pretty good test: http://enchroma.com/test/instructions/ That site is trying to sell you something, but my results there are consistent with others, e.g. http://colorvisiontesting.com/ishihara.htm
Neither can I, and I'm not color blind. I think it's just approaching the limits of human vision, in conjunction with your retina being "stained" by the previous picture (look away from the screen and back at it and you'll do better). Tetrachromats might fare better in this scenario.