Thanks to Stefan, Dan, and David for your replies. I'll look into all 
three approaches, but this looks most directly like what I had in mind.

My limited intuition seemed to suggest that since RGB is an additive 
color space, there might be a direct way to subtract a cast out of it.

  - John Mills

On Sun, 31 Jan 2010, David Gowers wrote:

> On Sun, Jan 31, 2010 at 12:50 PM, John Mills <> wrote:
>> I am printing a scanned image onto a slightly toned surface (heavy
>> watercolor paper), and I would like to precompensate somewhat for the
>> effect of printing onto this warm toned medium. How can I correct the
>> image before printing to approach the same colors as a print on white
>> paper, at least in the darker areas?
> I suggest trying this:

> 1. Create an image full of the color of the paper.
> 2. Look at the RGB values of the color you chose (my test color was
> 244, 242, 219)
> 3. Open up the 'Levels' tool. select Red channel, type the appropriate
> value (eg 244) in the rightmost field under the 'input levels'
> histogram+gradient. select green, type appropriate value (eg 242),
> select blue, type appropriate value (219), OK.
> 4. The canvas should now be completely white #FFFFFF / 255, 255, 255,
> and you should have an appropriate Levels preset stored to apply to
> your pictures; you may want to save the preset permanently with the
> '+' button next to the preset selector (after first selecting it)

> Note:
> This method will clip out detail of colors that are as bright or
> brighter than the real paper color. This is basically unavoidable
> according to your description of the problem.

> HTH,
> David
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