Here are a couple of before and after of what can be done with color shifted slides.
These first two are of the same slide of the Citrus Tower in Clermont, Florida. This is an old slide of my parent's from 1970.
Before: Color shifted red.
After: Color corrected as best I could. Looks good to me. A pro could do better, but I'm pleased.
Before: Train ride at Silver Dollar City, Branson, MO, 1974. Also color shifted red, but not as bad as the one above.
After: Lookin' really good.
This final one is just an after. I didn't keep a before scan of this one. I'm including this one because it's original slide that my Father In Law took around 1961 was just faded. Slowly losing all color, not color shifted like the two above. It isn't faded too badly, so with Photoshop Elements 4, I was able to make it look as if it was taken yesterday.
This is very fun, but also very time consuming.