Neat trick, pretty cool.
You can also tell they are the same colour by covering your screen where all the other tiles are (showing only tile A and B).
There is one I used to have where you had to find the black dot. I had it on my desktop for a little while untill it gave me headaches..
Its a black and white pattern, and wherever you focus on the white dots, the others a black. Focus on the black dots and they go white... very weird. Can never find that black dot .