Michelle and Michael
Those are excellent tools you've devised! I have both kinds plus another set of cards like Michelle's that are mid-value grey and another that are very dark. The latter are especially helpful when examining paintings and trying to distinguish the range of blacks and other darks that an artist may have used. I found that against the white cards, all of the darkest values can look pretty much the same, whereas, in fact, there is often a subtle, but important difference.
The only thing I would add to Michelle's excellent example is that in real life the value range is far greater than in photos (I know I don't have to tell you this). In paint we can often achieve all of the mid-tones and most of the darks found in human faces, except perhaps for the darkest ones which, depending on the lighting chosen, are not always present. But, we can never achieve the lightest lights and still maintain any chroma in them. Our lightest colour is white and it is bereft of chroma. Thus we must use artistry.
I'll go a step further and suggest that this fact is one of the reasons so many artists do not achieve the fullest sense of volume that they could. If one strives to render the mid-tones accurately and faithfully--and, let's face it, the mid-tones (and shadow-line) are often the most interesting parts of a painting--it will mean that the flesh will be too dark or it may end up metallic-looking. We just can't be perfectly faithful to the mid-range and still convincingly indicate the lightest tones. The one (mid-tones) must be down-played in order to make the other (the lightest-lights) appear correct, thus giving the impression of a solid three-dimensional form.
Anyway, this is too hard to put into such few words and I probably shouldn't even have tried.
All the best.
Juan