I am a Photoshop guy, and I am also interested in history, so I have done some digital restoration of old photos of Phoenix. And if you think the sky was yellow back in the 1970s, or that the colors Michelangelo used on the ceiling of the Sistine Chapel were muddy and brown, you have confused the degradation of the original image with what it originally looked like.
So, the first step of doing a digital restoration is to realize that Alfred Hitchcock did not photograph "North by Northwest" with glitches and scratches all over the place. And the faded color was caused by the aging process of the film. Yes, the sky was blue even then.
Unfortunately, Photoshop has a lot of tools that can make an image even worse. If you are not sure what to do with a scan, please leave it alone. Downtown Phoenix didn't look like the surface of Pluto, either. I've seen images that have been messed with in this way, and there really isn't much I can do to rescue them.
I use Photoshop Elements and the first tool I try is Enhance - Auto Color Correction. But not always. You have to watch what you're doing. There is also Auto Levels, Auto Contrast, Auto Sharpen and Adjust Lighting - Shadows/Highlights. But if you just click these tools blindly, you're gonna have a mess. I practiced a lot with this before I started using it to create the "high resolution archival quality digitally restored images" (as I call them) that I post on Phoenix, Arizona Historical Images. And by high resolution I am talking about a maximum width or height of 1600 pixels per inch at 72 ppi.
There are also various tools in Photoshop that can repair scratches and glitches. Again, watch what you're doing, or it will look "Photoshopped". I use the Healing Brush tool a lot. But it can do unexpected things - practice with it. There is also the Clone Tool. Very useful, and the most commonly misused tool in Photoshop.
I am doing this just for fun because I am a time traveler. When I look at old cars that have been restored, I want to imagine that I am seeing it in a showroom in 1954. Especially if it's a Corvette!