I know 18% grey cards are supposed to provide a neutral color to set custom white balance. But why are grey cards used instead of pure white ones?
My assumptions: Is a specific greytone easier to produce than pure white? Does white stain so much easier? (or Does grey just doesn't show stains that much?)
Answer
When you have already set your exposure parameters, white could be clipped in some single color channel (but not all, so your camera won't show it as blown), therefore not being very good basis for color balance adjustment.
Also, paper will turn yellow during time. And just looking at the sheets currently on my desk, there's three different tones of white papers already.
That said, I've used white paper many times and it will get you close enough in all but very critical studio shoots. Very often you'll have multiple different light sources, reflections from colored surfaces and/or shadows vs. lit areas, all having much more impact on colors than slight imperfection in metering the white balance. Just make sure you don't clip any color channels (e.g. by spot metering without compensation from the paper, or verifying exposure parameters using color channel histograms).
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