>I'm surprised that Photoshop doesn't provide a more facile way to compare
>color between images...it would be convenient if the RGB
>or CMYK numbers from the first image would stay up there on the screen
>somewhere, so you didn't have to scribble them down, when you went to the
>other image. I need to make some measures and comparison with great
>exactitude.

There are several ways to make such comparisons, it all depends on what you are
trying to accomplish. Here are some possibilities:

1
-
Make a selection of the area to be compared. Hold Cmd/CTRL+Shift
and click inside the selection, and drag into the other image -- the
image area will copy to the other image. Move the sample over the
area to be compared. Place a color sampler in the spot
and toggle the view for the layer to get the comparison between original
and other-image values while viewing the Info palette.

2
-
The foreground/background color swatches will hold a color for you.
Click on a color with the eye-dropper in the first image and then
compare it by rolling over the second image and the swatch in the
to see the sampled numbers in the Info palette.

3
-
If you have 2 versions of one image to compare, try this:
1.flatten both (not completely necessary, but humor me...).
2.hold shift and use the Move tool to drag one image into the other.
3.change the upper layer you just dragged in to Difference.
This doesn't give you numbers to compare, but it does show you what parts
are different -- using a pixel-by-pixel comparison. The brighter the
result, the greater the difference.

4
-
Take a screen shot of the Info palette and open it so the color
measures freeze on screen. You can actually measure 5 areas
at one time and freeze-frame the samples this way -- one for
each sampler, and one for the eye-dropper.

There are more.

I think the better question might be about what you are trying to
accomplish in the comparison. What is that goal? if you are looking at
individual pixels, and trying to get one color to match another, you
may be looking too hard. The goal of corrections
(if that is what you are doing) is to make the image look better. A
comparison between images assumes one is correct and the other isn't --
but even the idea of 'correct' may be gray...is a correct image one that looks
the best of one that matches original color?

The type of comparison above may be handy in some cases where
you really have to match color. Photoshop CS provides some means of color
matching which I have yet to really explore (there are far more interesting
things
in the package for me). if you are making scientific measures that's one
thing...
if you are making curve corrections to a graycard measure OK. Many times I
see people making lotsa fuss over matching a color between images, and there's
no real
reasoning behind the effort, and no guarantee that the color you are matching
to is the right one (if there is a 'right' one). Trying to amtch prints and
screen is a valiant
crusade, matching them between images just because you feel they should
may not take into account changes in the scene or other variable which
actually makes the difference desireable!

I think you are right about the measures, though. Would be handy to have
a quick way of freezing them for those times you need a measure (though
I look at these usually as samplings to determine approximations and
ball-park numbers). I've requested that before as well as a screen-shot
utility. I've obviously not got hold of the right adobe ear cause neither are
in CS. Some things I've suggested made it in, but who knows if it was my
suggestion or someone else's. Again, another fact I could research, but I
gain nothing from knowing. I like to be practical and put that kind of
incidental fact aside. it might be great exactitude, but it also might be
unnecessary.

Hope that helps!

Richard Lynch
http://ps6.com
http://hiddenelements.com