You can make an old leather couch look brand new using a leather repair kit or with products for restoring leather surfaces. Leather dye is also available, dying leather can restore or brighten up the original color, and in some cases, change it completely.
Leather Repair Kits
You've probably seen on television products like the New Liquid Leather Vinyl Fabric Repair Kit/=. These products have a liquid that consumers tint to match their leather, then apply to the damaged area. It bonds well with the leather & consumers report that the most difficult part is matching their leather color. It seems, however, that once the task of color-matching is complete, the repair is seamless and long-lasting.
For completely changing the color of leather, a leather dying kit may be in order. The Tarrago Self Shine Color Dye Kit has a good reputation for being easy to use. It only covers a small area, it's marketed for shoes. Luckily, it's inexpensive. With 2-3 kits, you could recolor an entire couch for less than $30. I think it would be fun to get the kit in a few different colors and make a wild pattern. The link I provided was for a color called "moon blue." Picture that swirled in with greens and purples, you'd have a one-of-a-kind piece of furniture for an incredibly small amount, especially if you bought the couch used. I've seen ripped leather couches at yard sales for next to nothing.
When you're trying to restore leather to its original color, you may want to do your color-matching in a discreet area, like underneath. Likewise areas that are tucked underneath the upholstery may more readily reveal the original color. Sometimes in upholstering, the leather will be tucked or folded and the areas inside of the creases, which haven't been exposed to sunshine or other cleaning products, are protected and preserved. In fact, professionals who restore antique furniture us a similar technique to attempt to reveal the original upholstery fabric for restoration. The Leather Luster Kit is a popular choice for leather restoration.
Give it a shot, those old leather barstools and couches might look hopeless and ruined at the thrift shop or yard sale, but if they're sturdy and simply in need of some "leather love" then by all means, give it a shot. What's the worst that can happen? At best, you'll end up with some jammin' new leather furniture that looks great for hardly any money and at worst, you'll end up with another something to add to your yard sale.