Non-destructive "toning" of monochrome images.
Start out with a greyscale step-wedge that goes from 0 to 100 in 11 steps. In other words, the Luminance or Brightness values should be exactly 0, 10, 20...100.
Using any method you like, convert the stepwedge to "sepia" - or any other color you want.
After conversion, each of the steps should still be exactly the same Luminosity as they started out, namely 0, 10, 20... 100. If we start with a step wedge that contains 100 tones, they should all be the same as they started. In other words, all the original Luminosity values should be maintained.
I have tried a variety of methods that are provided by Photoshop and GIMP, and which are discussed in tutorials on the web. So far, every method I have tried, results in a change to the luminosity values - including the Photoshop tools which provide the explicit option called "Preserve Luminosity".
(After using the standard techniques, one could always apply some kind of correction curve to undo the damage done, and manually "force" the Luminosity values back to where they started - but I'm looking for a procedure that actually does this automatically: a non-destructive conversion).
Please don't ask me general questions like: "Have you tried try LAB color ?", "Have you tried X ?" or whatever. I am looking for a method that works. If you have one, I bow to your feet in gratitude. I confess, I'm a beginner !
Thanks in advance.