Color Cast Removal

[Mike Sellers]

I have this old 4x5 transparency that has a strange color cast to the sky that I am trying to remove. It almost looks like a chemical problem from the developing process?

[Christoph C. Feldhaim]

Looks very problematic to me, since it seems to be a non-uniform artifact, be it from development or from storage/aging/fungus.
You might check in b/w mode if it goes through to the luminance information of the image.
If not you might use some fake color to repair it, if yes the problem is even bigger.