Photos (slides, film, photographic papers, etc) use layers of red/green/blue pigments in order to reproduce the colors in a scene; and when an image color shifts it is due to the different pigments fading at a different rate. In this case blue and green have faded more than red, although the red has faded some as well.
In order to fix this you have to add the missing colors back in; it is essentially a white balance issue, although it is not even/uniform. There are many ways of dealing with white balance issues; levels adjustment (setting white/blacks), average color inversion, color balance, etc, etc... lots of tutorials available on the web.
I used some of a lot of them, along with masking to limit them to specific areas... it's not perfect, but it was fairly quick.
And this is what the layers looked like (layer1 copy is an inverted average color layer)