This is another eye trick, like the color of the dress. Or is it a door, or the beach? This is very simple. As the square moves left to right, does it change color?
The guy who posted it says it does not change. This is a phenomenon we call lying. It changes color. Anyone who says it doesn't, um, I will fight them with a trident.
Allow me to be a...
Let's look at the block as it appears on the right side, in the "aqua" section.
It is obviously a pinkish color.
Now let's look at the square when it resides on the pink side.
Wait. Without the aqua color in our line of sight, it might be the same color.
Lets put these two together so the squares touch and see if there's a color difference.
Dammut! It doesn't change! My world is upside down.
I curse my own accurate detective work!