I don't follow you. Wilhelm's iStar is designed to give a numerical value to his permanence ratings, allowing a metric to track how a particular print changes over time.
That's initially why Henry started it, but its evolved far more. He's working on using it as I said, as a matrix for difference in how we perceive images.
Saying that Delta E values provide "a very simple measure of difference of a single set of colors" is misleading. They provide a very precise measure of difference over any number of colors.
Solid colors yes, but it tells us little if anything about color appearance nor is the color model its based on, with its various warts, based on color appearance models. Its useful, no question. But it has a slew of areas where its not to be used as a definite point of reference.
Probably one of the best posts ever on the issues are from the late Bruce Fraser. Note too, that the work Henry is doing is an attempt to provide better data for imagery:
CIE colorimetry is a reliable tool for predicting whether two given solid
colors will match when viewed in very precisely defined conditions. It is
not, and was never intended to be, a tool for predicting how those two
colors will actually appear to the observer. Rather, the express design goal
for CIELab was to provide a color space for the specification of color
differences. Anyone who has really compared color appearances under
controlled viewing conditions with delta-e values will tell you that it
works better in some areas of hue space than others.
When we deal with imagery, rather than matching plastics or paint swatches,
a whole host of perceptual phenomena come into play that Lab simply ignores.
Simultaneous contrast, for example, is a cluster of phenomena that cause the
same color under the same illuminant to appear differently depending on the
background color against which it is viewed. When we're working with
color-critical imagery like fashion or cosmetics, we have to address this
phenomenon if we want the image to produce the desired result -- a sale --
and Lab can't help us with that.
Lab assumes that hue and luminance can be treated separately -- it assumes
that hue can be specified by a wavelength of monochromatic light -- but
numerous experimental results indicate that this is not the case. For
example, Purdy's 1931 experiments indicate that to match the hue of 650nm
monochromatic light at a given luminance would require a 620nm light at
one-tenth of that luminance. Lab can't help us with that. (This phenomenon
is known as the Bezold-Brucke effect.)
Lab assumes that hue and chroma can be treated separately, but again,
numerous experimental results indicate that our perception of hue varies
with color purity. Mixing white light with a monochromatic light does not
produce a constant hue, but Lab assumes it does -- this is particularly
noticable in Lab modelling of blues, and is the source of the blue-purple
There are a whole slew of other perceptual effects that Lab ignores, but
that those of us who work with imagery have to grapple with every day if our
work is to produce the desired results.
So while Lab is useful for predicting the degree to which two sets of
tristimulus values will match under very precisely defined conditions that
never occur in natural images, it is not anywhere close to being an adequate
model of human color perception. It works reasonably well as a reference
space for colorimetrically defining device spaces, but as a space for image
editing, it has some important shortcomings.
In the years since the original 1931 specification there have been a number of revisions to the dE equation. The latest is dE-2000. which does a commendable job of treating color shifts equally across the visible range. This is exactly what we want when evaluating profile accuracy.
Better, not exactly what we want. And you know why all the updates....