[Rico]

In regards to adjusting white balance manually a white sheet of paper or white surface is needed. Starting on page 125 of the K10D manual explains the steps to manually setting white balance. It states: Under the light to measure the white balance, fully display a white sheet of paper in the viewfinder or select a white area as the subject. Using a neutral or gray card will not work when adjusting white balance manually as described by the owners manual.

It is advantageous in this circumstance to adjust the white balance manually for copy stand use. It is a custom white balance to the light source. It can be very effective in the field as well.

[SOldBear]

It states: Under the light to measure the white balance, fully display a white sheet of paper in the viewfinder or select a white area as the subject. Using a neutral or gray card will not work when adjusting white balance manually as described by the owners manual.

Note that the second sentence ("Using a neutral or...") is not from the manual.

As a poster wrote in this thread, for the purpose of white balance, "white" is the same as "neutral gray."

In fact, in my experience, a piece of a neutral gray card (the 18% used in the old film days) is better than a white sheet of paper. Why? Because when it's too bright, our eyes can not distinguish slightly different shades of white. Many "pure white" surfaces actually contain a tint of blue.

The gray card also helps in metering.

[ma318]

I have used both white paper and gray card to set white balance. They both work fine.

Pure white has RGB value of 255,255,255.
18% Gray has RGB value of 128,128,128

When you set the camera's custom white balance using either white or gray card, you are basically telling the camera that the RGB values of the scene should be all equal.
If they are not equal, then the camera knows how much to adjust/shift the RGB values to get them equal. (e.g. more blue or less blue, more red or less red, etc...)
Therefore either white card or 18% gray card would work for setting custom white balance.
In theory, if you print a card with any color with equal RGB value (e.g. 50,50,50), it should also work for setting white balance. However, I have not tried this myself.

But to set exposure, you have to use a 18% Gray card.

(I agree with SOldBear that, unless you carry a 18% gray card or pure white card with you, finding a surface you think is white indoor to set your custom white balance may be difficult. Your eyes may not be able to tell how white the surface really is under artificial lighting.)