Chapter I

Probably you have some mixed concepts.

• Not every gray card is neutral gray. A gray card could be just intended to measure exposition, middle gray, not neutral gray.

• The white point of a diffusive white is not meant to be 255 white on a photo. When you use the white dropper on the white that is what you are getting. You are forcing some part of the white paper to be white 255. As your intention is actually to be printed, it is ok. Just keep the lighting super uniform.

• A white diffusive material, as one sheet of paper, depending on the dynamic range of the camera could be around 75-85% using the same exposure that reads the gray card as middle gray.

• White balance is intended to nullify color casting variation of the main light, For example, remove the orange on an incandescent light.

• White point defines the brightest point of the file. Black point is the darkest.

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Chapter II

To answer the specific question. The best way to do this is

1. Use the gray card to get the correct exposure (or using an incident light experimenter)

Take a photo of the target, see the histogram and adjust the light, shutter speed, iso or aperture, so it draws a clear line in the middle of the histogram.

2. Use a color target, like the Xrite passport. If step 1 is done correctly this is now properly exposed to maximize colors and the dynamic range of the shoot.

3. Make a color profile of that photoshoot conditions using the x-rite software.

This will balance, not only the white balance but also small deviations from specific zones of color, and also will make fine-tuning of the exposition.

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Chapter IV

A cheaper way with the gear you already have.

A. Take a sample photo to be used as a white balance. There are two ways or logic to do this.

A1. Take a photo of your light source. Of course, it will be very bright.

• Lower the ISO to the minimum.
• Increase the shutter speed
• Change the focus to manual and de-focus it so you have a big blur.
• Michael C recommends a gadget on the comments that helps you defocus everything to have an average blur.
• As I always use soft, diffuse light I can close the aperture to the minimum, and still take a photo of a big blur.
• But if you are using hard light, open the aperture, to combine it with out of focus to have the blur.

The objective is to have the histogram of the light source not at full white but inside the histogram, let's say at 70%. This will tell the camera that that is a white-balanced light.

A2. If a reasonable white is dominant on the shoot, for example, a white seamless paper is a background, take a photo of that background to define the white balance. Probably this background is not pure white, but when seeing the photo, our brains will want it to be, so use it as such.

B. Define the correct exposition using the grycard as I mentioned in point 1.

C. Compensate to your taste. A middle gray defined with point 1, will render photos "too dark", especially for a high-key photo. You can based on your preferences augment, let's say 1 stop. But try to do this on post-production.

D. Remember to shoot in raw.

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Chapter V

The eyedroppers tools in Gimp

Here is the test file. It has an upper ribbon of neutral grays A and a lower band with an orange color cast B.

1. On this screenshot, I have an external tool. It is a color picker to take additional readings.

2. Using the gray color picker on more or less where the orange line is, changes the file as expected.

3. The orange cast on band B is removed in general, the A band now has a blue cast, and the reading with the external tool indicates a neutral value.

4. Resetting the Levels dialog box, and using the white point tool on the neutral band A more or less on the spot 5 It only clips the white.

6. But, resetting the values, and using it on the B band it does not only clip the white but also changes a bit the white balance.

7. A band It has a bit of a blue cast, but evidently the orange cast on B band is not removed.

Using the two eyedroppers together, but in inverse order seems to have different results.

8. White point first, gray point second = Blue cast on the targeted zone.

9. Gray point first, white point second = A slight orange cast on the targeted zone.

The tests are not super precise. But probably a good option is to use Graypoint first, and Whitepoint second.

But keep in mind that the white point artificially clips the values.

I do not currently have Ps. If someone wants to test this target file, and comment, it would be helpful.

P.S. Probably a pointless rant. The tool "gray point" eyedropper probably should be renamed "Gray balance". n_n

Additional reading:

Do I always get the same colours when I set the white balance correctly?

Color issue: studio images have a pink hue

Photography Stack Exchange questions tagged White-Balance