Thanks Neil!
Unfortunately, this very short description is very difficult (if not impossible) to follow and thus only of limited help for non experts like me.
If I understand Ann Bens correctly, the effect is only on a Mac removed. Is there a good reason for this? Are there help videos specifically for Mac users? Are Mac users not important enought for Adobe?
The description is quite precise.
After Alt/dragging the clip to V2, then again to V3, you have three copies of the clip stacked.
Using the Lumetri panel on the top clip, you go to the HSL tab, and use the mask or key controls to select the brightness, hue, and saturation of the color you want to work with ... to modify. Set the HSL mask to "color/black" ... and leave it on (active) so you see the image with the color you want showing, the rest of the image black.
Now go to the clip on V2 ... apply the Track Matte effect, which has two main options. as noted set the pull from option to the track above (V3 in this case) ... and the other to Luminance.
Now again on the clip on V2, go to the Lumetri panel, and any control in the entire panel will work on ONLY the image data passed through from the mask on V3. And the results will be applied to the original image on V1 and show in the monitor. You can even use multiple instances of Lumeter on the clip on V2, but they all must be after (below) the Track Matte effect on the effects stack for the clip on V2 (check the Effects Control Panel).
I've at times had up to four sets of the V2/V3 above working on a clip.
This is also the best method when you need to pull a key for a correction/change, but want to ensure you pull from original clip data so no other mods to the image change that key.
Neil