Probably not a good idea to be rushing out to buy leather dye. Before you do anything, take a good look at the leather and decide what you want to use it for.
Is the finish worn through where the tush was sitting on it in its couch days to the point where the finish is worn off? If so, then dye is just going to turn it very dark and it'll run.
Can you arrange your patterns on it so that the wear is in the right places? For example, on a hunting bag you'd like the backside that rubs against your shirt to be as close to fully finished as you can get it so it does not rub off on your clothes and the seams should not be in a worn or fuzzy area lest they give out.
Are you going to use one single layer of leather for your project or can you line it...say, for example, a hunting bag or belly bag. I made my last two hunting bags from worn deerskin lined with light canvas and the bag is not only durable but also avoids being shapeless or flimsy looking.
A little gluestick will hold the cloth in place so you can stitch inside out and the turn it right and the edge of the cloth is hidden by the folded seam. A seam is where most of you wear takes place except for something like the heel or toe of a shoe.
Is there too much shine? That's a give away for a 20th century piece of leather. A de-glazer will take off the plasticky finish for you unless the stink kills you first.
Try some oil or paste on the worn areas first in a scrap piece. Sometimes you can be surprised with the results and you may not need any dye at all. You might also want to try brisk rubbing with a smooth glass jar where there are no ridges. The leather you burnish this way will have a different appearance than anything you apply in liquid form.
Olive, Neatsfoot, mink or even motor oil will work as long as you don't rub in anything that has silicon on it. You can't beat lanolin paste for that. Shoe polish will wear off, and that's why shoes have to be periodically shined.
One craftsman I know uses oven cleaner to age his leather. I pray for him occasionally.
Lastly, don't kill yourself on the finish only to betray your work with poor stitching. Stay away from the rotary punches and engage a polished awl and anything other than artificial sinew. Try to keep your stitches between 1/8 and 1/4 inch apart...the former for thinner leather and the latter for thicker. That seems to be close to the standard for 18th and 19th stitching on everything except shoes.
Yeah, it is kind of ironic that the shoe repair guy ( a cobbler) survived the shoe maker guy ( A Cordwainer) and the lesser guild had the endurance. Most of those guys today, tho, do know a lot about leather, and can help you with your projects. Let your own judgement prevail, tho.
Good luck.