How can you repair a worn and scratched up leather couch? The color of the leather came from a dye that was mostly on the surface. Abrasion (mostly from chihuahuas!) scraped off the color and left a rough scattering kind of surface that mostly looked gray.
Of course the original pigemnts used when the couch were made are not available, not is there any kind of record of the composition of the dye.
Luckily we can match colors, by mixing pigments in such combinations that the mix appears to have the same color as the original substance, even though the intrinsic property of spectral reflectivity may be differnt.
The reason is the eye is essentially a 3 color sensor, and the net perception is a combination of the signals from these 3 color sensors. The sensors have broad response, so that all the color sensors can sense light across the visible spectrum, but their sensitivity is optimized for a particular color.
We sent an undamaged piece of leather to a firm called leather Magic in North Carolina. They belnded pigemnts to match the color values of the test piece.
The result (after some exhausting cleaning, prepping and coloring) was truly remarkable. The restored leather matched the leather sample exactly (as determined by eye).
The spectra analysis shows something differnt! In fact the reflection spectra of the new pigments and the old pigemnts are significantly different. As shown below, the new leather has a much higher NIR reflectivity, and a different shape in the red region. The two small peaks are in fact from two pigemnts that were belnded to match the color of the original single pigment.
The NIR spectra are also intersting from a heat budget perspective. When exposed to sunlight, the couch warms up. The higher NIR reflectivity might result in a cooler couch.