How to compensate for faded slide photos

Which system version is running on your iMac?

Try to find out, which color is making the slides look unnatural. The pigments of the different colors are decaying at different rates. If there is eg. to much green in the color or the slides are too purple, try to use the "Selective tour" tool in Photos.

For example, this slide with two raccoons has been damaged by light at the border of the slide. At the lower border was too much green.

I used the eye dropper to sample this green area and lowered the saturation just for this color. Before:

After:

You will notice, that Photos removed the green everywhere. Now the foliage is also no longer green. Photos does not have any brushes to adjust the color locally. I do not mind, as the photo is a night shot.

If the unnatural colors are just slightly unnatural, it might to suffice to use the White balance tools. Look for a spot in the image that should be white or or gray, and sample it with the eye dropper in the White balance tool. Photos will adjust the lighting to make this spot gray.

You could also try to adjust curve for color component that is giving a color castand try to tweak the color curves.

If all fails, use a third party tool or turn the photo into a black&white image. This might be the better solution, if the photo has stunning lighting effects. I simply used one of the built-in Filters to make the photo black and white.

I just did this with some photos where the green and blue channels were badly faded and pretty much only red remained. I found the levels tool very helpful:

Apply levels adjustments to a photo in Photos on Mac - Apple Support

Just to repeat léonie's recommendation-- The Selective Color Tool is a great device for fixing scans of slides. (Ektachrome, too, fades funny, as do, I think, Fuji, Agfa, and all the Ektacchrome-type films. Kodachrome is way more stable!) It's almost always the Selective Saturation that I adjust. Often, the slides can use a bit of warmth from the White Balance Tool, and I sometimes use the color-specific Levels, though that seems cruder. I almost always have to adjust Options>Saturation and Levels, because color intensity and contrast gets lost when adjusting other things.

Each time you adjust something, you're losing a bit of information in a trade-off to see something else better. At some point the loss becomes too great, and you just do the best you can.

The RG&B Levels are really useful, but you must try out the Selective Color Tool with the dropper. Fast and amazingly versatile! I adjust Saturation, mostly. It works well for fluorescent lighting, too.

You're welcome, Amer.

A Happy New Year for you too 🍀 🌟

One more thing - for many of my old slides it helped to use decent scanner software.

VueScan has many presets for the different kinds of film. I got much better results right out of the scanner with VueScan than with the scanning software that came with my scanner.

Ektachrome and Fujichrome tend to be greenish. Kodachrome was warmer, and it made lovely pictures. Unfortunately Kodachrome was slower, and only Kodak could process it. I kinda miss film.

(I hate to admit it, but digital pictures are really better....)