I have the artistic talent of a small potato and have almost no knowledge of these things however I read that GIMP is in some cases a photoshop competitor, and free.
https://wp-modula.com/free-photoshop-alternatives/
Of course as I am sure you know the faded photos have lost information that cannot be recovered. Colour prints are particularly susceptible to damage, I suspect it's UV that does it. The negatives would likely be OK. I don't mean that skilful artistry or magic software can't improve things, I sometimes brighten or dim photos with the Windows Live Essentials photo program for example.