Some of the best comments so far have come from a community blog called metafilter.
One of the members of this very smart community presented this illusion in which square A and square B are the exact same shade of grey.
Other comments include:
"When painting in Photoshop I find I have to set the little swatch to what looks like a darker color than I need. Large areas of the same color look lighter."Great stuff. I've never heard of qualia. There are also mentions of Dan Flavin. I had never been exposed to Flavin, however I see the connection.
"This is why it is important to rough in and develop your painting as a whole, and not to zero in on one part for detailing. If you do that, you may be in for a color surprise when you fill in what's around it."
"The question is whether qualia exist, whether the subjective gut feelings of the taste of chocolate, the color red, the feel of your favorite sweater - have some intrinsic existence or only have meaning relative to other perceptions."