Reader Julie wrote in with this really challenging color test. The object of the game is to arrange the blocks of color within each row so that there is a clean transition from one color to the other. 
It gets really hard when you get down to the nuances of one color to the next, and sometimes difficult to decipher the differences depending upon your monitor and unregulated lighting conditions. 
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There is also a non-virtual version of this test for those looking to improve their ability to discriminate between colors. They even suggest a daylight viewing booth to help see each color accurately by regulating the viewing conditions.
It gets really hard when you get down to the nuances of one color to the next, and sometimes difficult to decipher the differences depending upon your monitor and unregulated lighting conditions. 
image source
I'd like to know what it is supposed to look like on a calibrated monitor, but here's what I got when I tried the test. 
Give it a shot! How did you do?