You are provided an empty grid with data on the x- and y-axes for the distribution of colors within the grid. Then use your mouse to fill in the blanks.
I thought the 20x20 grid was a difficult (but fair) challenge. Found by Alex and posted this morning at Neatorama.
The game is here.
Addendum: A hat tip to reader Muzzman, who knew of a similar (and free) site at Griddlers.