So I have a PSD in which I want to put my friend's face on to "The Situation's" body (Jersey shore yo!)
I want to color correct the skin tones to match, so I use the eye dropper to sample the color of each layer and i get the RGB value in the info palate for sample #1 (friend) and sample #2 (guido).
So I want to CC my friend and I apply levels or curves but when I do so the RGB value for Sample #2 disappears in the info palate. Now I know I can just write the value down but why doesn't it just stay up so I can tweak the RGB values in #1 and see #2 right there!
The color sampler does not remember the layer you used when you created a sample, it only keeps track of the location. As you change layers, the value of where you color sampler will change to show the color of the visible layer of the3 layer you have selected + the layers underneath that one.
You say that colors are disapearing, that makes me think you are using image >> adjust >> curves, rather than the curves adjustment layer found in the bottom middle of the layers palette. Put all your color adjustments layers in a folder and put a layer mask on that folder, should give you a good image.
Also look into using image >> adjustment >> match color. This usually works good, unless there are other totally different colors in the background throwing off the filter.
Post the image when you are done so we get a peek.