Explanation: you just selected all completely black pixels (0,0,0) i.e. all pixels that are identical in both layers.
Explanation: the selection now covers all the other pixels, i.e. all pixels which are different between both layers.
Explanation: you now see all identical pixels in their respective color and all different pixels in white.
What Color Checker 24 (CC24) you are using with DNG profile editor? There are two CC24 available. They are Macbeth and X-Rite. These two CC24 have different RGB valued for color and grey patches. I have calibrated my camera with X-Rite CC24 then read back the RGB values and non of them was close.
"Dark fade" is more properly called "thermal fade" or "thermally-induced fade". And it doesn't just occur in the dark. It refers to the fading reactions that are initiated by heat and occurs even while an image is in an illuminated environment. Along with light-induced and thermally-induced dye reactions, the other two of the Big Four are humidity and gas fade (fade due to to pollutants in the air, largely ozone these days, but also sulfur oxides and nitrogen oxides and others in special environments). To make things more complicated, these four factors can interact, sometimes in a non-linear or non-additive fashion.
The first company to advocate testing all four factors was Eastman Kodak who also completed the only worldwide environmental study to determine how these factors should be balanced in typical keeping environments. Wilhelm Imaging Research, which in the past had ignored everything except light fade, made predictions for the image lifetime of some products that turned out to be completely bogus because of unaccounted for ozone-induced fade. The WIR website still has reports of contracted tests for some products that say ozone "Test in Progress" years after the first posting (ozone tests are amongst the quickest to perform).
While all materials will undergo thermal fade, good printed materials like color charts tend to be quite resistant and thus have a long lifetime if kept out of the light. Depending on the precision you require, most people can count on color charts stored in a dark, room temperature environment and away from pollutants to pretty much last a lifetime.
Inkjet materials using pigmented inks also have excellent thermal keep, probably in excess of 100 years for the best materials.
Thermal keeping at its best is not a linear function of temperature (it is governed by the Arrhenius equation) and a very. very rough rule of thumb is that thermal fade reactions double in rate for every 10 degree C rise in temperature. Thus, the colder you can store your images the better.
Also, I've referred to thermal "fade". Actually, a better term is degredation as some materials do more than fade; they yellow (that is, increase in blue density), usually most noticeably in the white areas, but the same yellowing is occuring in imaging areas as well, thus changing the density of color patches.
More than you wanted to know, but like so much in life, this is a complicated subject.
Message was edited by: PhotoSci: corrected typos