Color subtraction HELP needed

Hi, I am trying to figure out how to automatically subtract a color from an image. I created a model so that i can play with:
http://img49.imageshack.us/img49/5918/testremovecolor2wm.jpg The original solid ping wide rectangle was superimposed with a 50%transparent light blue rectangle.

Now, is there a tecnique to "subtract" the blue rectangle and get back the pink one? I am playing with Apply image and Calculation with no success.

Of course this is an artificial example, but if i understand this i can apply the method to othee real photos.

I use Photoshop CS2

thanks Dario

In article <dtf1j9$im4$>,
Dario wrote:

Now, is there a tecnique to "subtract" the blue rectangle and get back the pink one? I am playing with Apply image and Calculation with no success.

You can not make the blue go away and get back the pink, because *there is no pink beneath the blue.*

You can use Replace Color to replace the blue with the pink, or you can use Select->Color Range to select the blue and fill with pink.

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tacit wrote:

In article <dtf1j9$im4$>, Dario
wrote:

Now, is there a tecnique to "subtract" the blue rectangle and get back the pink one? I am playing with Apply image and Calculation with no success.

You can not make the blue go away and get back the pink, because *there is no pink beneath the blue.*

You can use Replace Color to replace the blue with the pink, or you can use Select->Color Range to select the blue and fill with pink.

In this example, it'd be easiest to select the rectangle and fill it with the pink. But to, say, remove a color cast (which is, I think, what the OP is getting at), you'd want another solution.

There actually *is* pink beneath the blue (sort of). I use LAB, but within any color space you could use a curves layer to turn the blue into pink, and then layer Blending Options to isolate it. I did it in less than 5 minutes, and I'm not especially talented.. It's easier in LAB than, say, RGB, but still doable either way.

Start out with the Color Sample Tool, click on the pink, click on the overlap. Make a curve layer to change the overlap color to the pink color. Go to Blending Options and use 'Blend If..' to let your curve layer shine through where it's wanted, and not where it isn't.

Voila.