One of the characteristics that really distinguishes old photos is the brownish tone they have. It's called sepia, and it comes from dipping the finished print in a bath of squid ink and water. Sepia toning was done to give the print a rich, warm quality rather than the original drab gray. This process also had the effect of stabilizing the print, making it last longer.
I find it harder to work on sepia-colored pictures, so I always remove the color before I start editing them. It's easy to replace it afterward. You'll learn how later when we talk about duotones.
There are several ways to remove color. Probably the easiest is to use the Remove Color command that we discussed earlier in the chapter: Enhance, Adjust Color, Remove Color, or Command/Ctrl-Shift-U. You could also convert the picture to grayscale, which would have the same effect, but doing so would require you to convert back to RGB mode if you wanted to add back the sepia when you were finished. Another option is to open the Hue/Saturation dialog box, and move the saturation slider all the way to the left to completely desaturate the picture, again removing all color. All three methods produce the same results; I typically use the Remove Color command if I plan to restore the sepia later, change to grayscale if I don't intend to use the sepia again, and use the Hue/Saturation command to put the sepia back (although you can use this command to remove the sepia as well).
Removing color also can get rid of coffee and tea stains, colored ink smudges , and many of the other things that get spilled on a photo. Even if it leaves a gray blob; that will be easier to cover.