We held an annual Halloween party for 38 consecutive years. Lately I've been restoring the older photos.

I have the color balance in pretty good shape now, but the some of the photos appear to have suffered some kind of oxidation in the dyes. The result is that the edges are looking pretty greenish, and some also display greenish blotches (see provided example).

What I'd like to do is somehow gradually REMOVE the G component along the edges of this photo with the airbrush tool. Or more generally, "paint out" any color in this manner, once I figure out the exact color that will achieve my objective. After a little messing around with the Level settings it seems to be pretty close to straight green, though.

Asking the experts here on how I can go about doing that.

Thanks in advance,
Bruce

Edit: Apologies, not answering the question about green edges. More of a general approach

Try the gmic plugin http://www.gmic.eu

Either the color curves or as this one of the mixer filters.

Just out of curiosity I tried it out.
Basically it was used in the first image (Top) Adjustment in the color casting followed by the use of the Mixer Channel.
In the second image (Below) the opposite was done - Color Mixer with subsequent adjustment in the color casting.


https://imgur.com/a/RxGxCCn
But I don't consider any of these as ideal procedures as I got them by luck in a trial of hit and miss.

Some pretty impressive results with the color mixer. I'll try that out, and thanks!

Bruce

Did it in a couple steps!
Inverted colour in Gimp
In GMIC I unpurpled it (magenta is opposite of green)
Inverted the colour again and the green was gone.

Well, that's definitely an improvement!

What I've been doing is Decomposing the image into RGB layers, painting into the Green layer with the airbrush (to darken the areas that are stained), then Recomposing. If it looks better, I keep it, otherwise undo and repeat.

The problem is that I can't see the effect my airbrushing is doing on the original image in real time. There has to be better way.

Bruce

Which function in G'MIC, do you have a screenshot?
I'm asking because at Repair/Unpurple , I don't have any magenta or green slider...

Sally, great find! I didn't know this filter existed. So far it seems to be the best solution found, thanks for sharing!

PixLab, just use the default values on the previously inverted image, and the effect will act on the purple areas. Then invert the image again. (basically the procedure said by Sallyanne)

The filter is the same as your image (Repair - Unpurple)

Thanks Krikor, indeed with the default value it works
Nice trick Sallyanne

Correct Krikor, I just used the default because magenta is similar to purple in that it is reddish.