Accurate Colour with Grey Card

I want to photograph a painting and achieve accurate colour. How do I do that using a grey card and processing in PS?

If you are shooting jpeg, you need to use a Custom White Balance. Look it up in the index of your camera manual (either "Custom White Balance" or "Custom WB") for specifics on how to set this up.

Alternatively, you can shoot in the Raw format, which gives you freedom in doing color/White Balance adjustments and corrections.

Still, even with Raw, it can give you the best "starting point" if you use a Custom White Balance or a preset in the camera for the conditions. Or, a good practice would be to put your gray card into one shot, and then in your Raw processor use the White Balance eyedropper to "nail" the White Balance.

But for jpegs, and even with Raw to start with, use a Custom White Balance for the most reliable results.

Thanks Tony. So can I use the painting for the sample in Custom White Balance or should it be a white card?

For a Custom White Balance or using the eydropper with a Raw file, you need a "neutral target", so use the grey card or a good neutral white paper will do as long as you use it with a "Standard" exposure so it comes out "grey", not a blown out white. Part of a painting will likely not be good! Not neutral!

What do you mean by Standard Exposure?

markimage wrote in post #14638065
What do you mean by Standard Exposure?

A "standard" exposure means to meter the "target" so that it is exposed to "medium", with the meter needle centered and using no Exposure Compensation. This will expose a White target to be gray.

So take a white/gray target and fill either the center "circle" in the viewfinder with it (using spot, center or partial metering), turn off Auto Focus, use a low ISO, and take the shot -- the white will be exposed to gray, and the gray card will also be exposed to be gray. Note that a "standard" exposure does not equal a "correct" exposure (exposing white to be white would require Exposure Compensation or using Manual Exposure).

Take the shot and use it for your Custom White Balance. Again, refer to your manual for specifics on the technique.

The reason you use the "standard" exposure is to that you don't want to "blow" the highlights, introducing a "false" color balance. Use a low ISO so you don't introduce noise from an underexposed "dark gray". Use Manual Focus so that the camera won't try to focus on a flat gray subject.

Fantastic. Thank you very much Tony.

Just a brief note to point out that the procedure outlined by Tony is the best you can do if you have only a grey/white card. This will account for any overall color cast caused by the lighting and will nail one color (grey) among the possible millions in an image. A photographer who frequently needs greater color accuracy will carry the process further by photographing a card containing number of color patches whose exact chromacities are known and constructing a profile that describes the difference (delta) in the way the camera renders those colors. As I said, as a rule only someone who needs a high degree of color fidelity in their work will invest the time and expense involved. For the rest of us Canon made generic profiles that are incorporated in their jpeg Picture Styles and in DPP and so did Adobe for their software; profiles that produce acceptably close to reality colors.

tzalman wrote in post #14638247
A photographer who frequently needs greater color accuracy will carry the process further by photographing a card containing number of color patches whose exact chromacities are known and constructing a profile that describes the difference (delta) in the way the camera renders those colors.

X-rite do a selection of products that make this process fairly easy. Their Colochecker Passport is a neat solution.

tzalman wrote in post #14638247
Just a brief note to point out that the procedure outlined by Tony is the best you can do if you have only a grey/white card. This will account for any overall color cast caused by the lighting and will nail one color (grey) among the possible millions in an image. A photographer who frequently needs greater color accuracy will carry the process further by photographing a card containing number of color patches whose exact chromacities are known and constructing a profile that describes the difference (delta) in the way the camera renders those colors. As I said, as a rule only someone who needs a high degree of color fidelity in their work will invest the time and expense involved. For the rest of us Canon made generic profiles that are incorporated in their jpeg Picture Styles and in DPP and so did Adobe for their software; profiles that produce acceptably close to reality colors.

^^^

The issue of color accuracy is not merely WB value, which is only one characteristic.

Here I have portrayed same WB value in all six frames:

1. one with nominal values,
   
2. one with reduced Vibrance,
   
3. one with unbalanced Red,
   
4. one with unbalanced Green,
   
5. one with unbalanced Blue,
   
6. one with unbalanced Magenta
   

Thanks all. I usually use Picture Style - Faithful, would this be the best choice for this subject?

Either faithful or neutral I'd say.

This book will help you out a lot - Light, Science and Magic. Lighting methods can make a big difference.