If you squint your eyes where they're barely open, then block the two white squares to the left of the "A" square (using fingers is easiest,) you can see "A" and "B" squares are the same shade of gray.
When you see a cartoon showing an artist holding up his thumb, what he's doing is either making a handy frame for his composition, or blocking out a distracting element in the background.
Kinda like this:
Well, not exactly what it means here, but you get the idea.
Les
