You have two color photos of a house built with redwood- a and b, exactly the same, except in b you made the sky blue, a was overcast. You want to demonstrate to someone that the red in redwood of the house is identical in both photos. Is there someway you can target the same spot on each photo and get the rgb or cmyk readings. I have tried using the eye dropper tool, but it is amazingly hard to pinpoint the exact spot.

If you put both pictures into one document (on separate layers but on a large enough canvas that both are completely visible), you can then put color samplers at the spots you need to compare. The exact readouts for each sampler will be visible in the info palette, hopefully making comparisons easier. I hope I understand your problem and that this helps!

Similarly to what FrannyMae said -- if the photos are exactly the same then put them in the same file on different layers. Add a color sampler to the sky and one to the wood area and toggle the visibility of the top layer on and off. The sky color sampler readout should change but the wood color sampler readout not.

Or (low-tech) print out the two photos and cover up the sky area on both of them.

No one mentioned that the eyedropper can be set for point, 3x3 or 5x5. The latter settings are less critical for finding comparative settings. The point mode can make finding exact pixels difficult except on very smooth textures.