In a thread about evolution, I think you'd be better asking when the Black turns White. After all, evolution claims that we can start with one species (black) and end up with a completely different species (white).
Now, as we go down the image from the top, it's like each row of pixels is the child of the row above it. We can clearly see microevolution taking place, as each row is slightly lighter than the row above it.
But [bless and do not curse][bless and do not curse][bless and do not curse]! Lots of microevolutions don't equal a macroevolution (which would be represented by the pixels eventually becoming white)! That means that no matter how pale the pixels get, they could never become what we would call white! And yet, there appears to be white pixels at the bottom row of the image! This clearly means that there are no white pixels, and what we think is white pixels is clearly an optical illusion!
Thus, by analogy with evolution, I have proven that there is no such thing as white.
(Or perhaps yuou'll realise that evolution can turn Species A into Species B over many generations in the same way. There is no single generation where it stops being Species A and suddenly turns into Species B, in the same way that there is no row of pixels where it stops being black and becomes white. Rather, each generation of animals is slightly less like Species A and slightly more like Species B, just as each row of pixels is slightly less black and slightly more white. Thanks for such a neat demonstration of how evolution actually works, AV! Didn't think you had it in you!)