When you think of photos that need restored we often think of the really old ones that are so fragile you can barely touch them without some chunk falling off. And those photos are great to restore however there are also photos within our own lifetime that are aging yellowing and fading and those images need to be preserved as well. The processes that you put a color photography through is a little different but a lot of the same techniques apply. These techniques can also be applies to the images you are taking today.
The first technique I want to focus on is called white balance. White balance is balancing the colors in an image so that your whites become white. This white balance is influenced by many things for example the color of the light when the photo was taken. Outside natural light has a different color hue than artificial inside light. The flash on your camera has it's own light color as well. In older photos the whites are not only influenced by where the photo was taken but also by the age of the photo. Black and white photos fade and yellow at a different rate than color photographs and different era's also have their own yellowing styles based on the chemicals and papers used for printing at that time. Color photographs fade and change more rapidly than black and white photos just based on the chemicals used to make the colors which is why as a photographer I make sure to print a few black and white images that will not shift color as quickly.
When you have an image that you are restoring one of the first steps is to alter the white balance so that you now have white colors back into your image. I look for items in the photography like white shirts or papers. Neutral grey can also be helpful, sometimes even blacks can help you know what direction to go to create a better white balance. If you don't have any solid blocks of color that you know are either a white or a neutral grey you can sometimes use the whites of the persons eye. It's a small area but if you focus on it it can help you figure out a good white balance.
The way you adjust the white balance is with the sliders on the side of most editing programs.
If I slide my slider to the right towards the yellow temperature area then my image becomes warmer and gets a yellow hue.
If I slide it to left towards the blue temperature side then my image becomes cooler and more blue.
As you can tell the ideal white balance is somewhere in the middle. For this image it is closer to
Which gets me closer to my desired result then I add the tint slider and fine tune my imaged till I get a balance I am happy with. In this case I added a little more magenta to my image to get the balance I wanted.
Now that I have fixed my white balance and got rid of the yellow cast on my image you can already see a huge difference. After that it is just a matter of playing with the rest of the manual controls until you get a result that you are happy with. Hope this helps!