Each Expodisc is (was?) individually filtered for correct percentage transmission, color balance, and even diffusion. Mine haven't shifted color balance in well over 20 years of use, and they also act as a lens cap. It's used in the same way as an incident meter, and can be used for pre-exposure (i.e. flashing film to a threshold value). It's basically turning your camera into a TTL incident meter with the addition of providing a reference for correct color balance. An Expodisc is much easier to carry along and lasts far longer than a gray card, even the plastic gray cards that I've tried over the years. An Expodisc is also much easier to illuminate easily at the correct angle, whereas gray cards often need to be more carefully angled and placed in the light.

White balance is the same concept as color balance, but appears to be used in a much more digital context, or by those who grew up on digital and don't know the more traditional film nomenclature. It's also typically aimed at balancing on a highlight (white reference) rather than a mid-tone (gray card), although I'm aware that it can be used on any neutral tone. Anyone who understands either system should be able to make the necessary mental adjustment easily.

I've not seen white balance referred to as a way to determine exposure, and that alone makes it very different from a gray card. White balance I've only seen used to set a color standard, and a gray card is often used both to set and exposure and color balance. Bleached white paper coffee filters are also often recommended over the lens for white balance in the same way as the Pringles lid, but I have not seen a reference to anyone using the filters for incident exposure metering through the lens. Has the Pringles lid (as I understand it production has changed from the one used for white balance) ever been recommended for metering as is, or with a specific compensation factor?

Lee