Not a do it yourself job. Call in the pros, Leather Medic.

Here is a link that might be useful: Leather Medic Website

There is a means of touching up the color, but unless you're proficient at using sprays and mixing colors...

There are leather spray dyes available at shoe repair shops under the name of Nu-Life, and offer a variety of colors. I'd call around first, as not all shops carry these dyes.

There are other leather dyes available too, but I found the sprays the easiest to use and offered the best result. Just remember less is best...

Here is a link that might be useful: Leather Forums

Call a professional and get a couple of quotes. You might be better off taking the cushion to them because these guys usually charge to come out to make a quote. Call and ask. I know because I used to repair leather. The colorant costs about $75 a quart and you have to mix it to get the right color. Also, there are different procedures for diferent types of leather. This is not a DIY repair, but if you want to try, there are several sites on the web that will explain how to do the repair and sell you the kit.

This is an example of why it is better to buy a sofa that has seats, backs and arms that are removable. When something like this happens you can order a new case from the factory.