The Island of Color Blindness (bbc.com)

the color blindness tests are rather pleasant to look at.

http://cdn2.doutissima.com.br/content/uploads/2014/12/teste-...

There's an app called percolate which operated on a similar principle.

http://mathematica.stackexchange.com/questions/40334/generat...

and here

http://www.percolatorapp.com/