I'm going to try to put you off trying this , read on and you will see why.

I have dyed leather when needed but it's quite hard to dye finished and used veg tanned leather, the leather will have a good fat content that makes the dye not fix properly but that's not the main problem with trying it at home.

To do it properly you would need to strip the fat out of the hide and get right down to the fibres, something you have to use specialist products for and best left alone. I also believe it damages the hide right down at the cellular  level but that is my own opinion.

If you were going to attempt to dye the hide yourself at the very least you will need gloves, breathing apparatus and overalls as some leather dyes are aniline based.The breathing apparatus can be an industrial filter in a mask.

I cannot stress enough how dangerous aniline dyes are. 

The fumes are the main threat and any dye that splashes onto bare skin will leech into your system,whether you scrub your hands or not it will get in to your body eventually.

That is why I DON'T do any now with aniline based dyes.

Read up on aniline dyes, make your own mind up HERE

Appearance of the finished article is very varied too, you can't really go to a lighter colour, only darker.

As it's veg tanned leather and has fat in it you can and probably will get blotches where some dye will not penetrate in to the hide as well as in other places.

If you decide to dye to a black it will have a blueish tinge and not be true black. 

Overall matching to the original colour is very hard too, you may end up with a two tone saddle if you try to dye the flaps back to brown or black where the stirrup leather rub for instance, then you will have a saddle that looks awful and you'd then have to go over ALL the saddle to TRY to match it all up!

Dyes tend to mark sweaty and wet horses, people's jods and hands and if it is aniline dye  it leeches into the skin on contact, into your skin AND your horse's so please do reconsider about dyeing leather.

You can get a fixer called resoline to stop the dye marking everything it touches but that makes it very shiny like Patent leather.

Over oiling to darken leather (not something I really advise as it can rot stitches and eventually oxidises it, therefore weakening the fibres and cell structure) can darken leather considerably, it won't be an exact match but quite close.

To be honest with you, I'd advise against dyeing leather unless you can be 100% sure the dye in not of aniline base, look at the ingredients BEFORE you decide to buy it and investigate the dangers thoroughly first.

Try a test area where you won't see it if possible, wear protective clothing and breathing equipment and keep all dyes away from any naked flames.

To try to colour leather back to what it was I use water based stains that we use in the trade, there are acrylic stains too which are ok.

Alternative stains (not dyes) are that you can make yourself and dab on to see if it makes any difference are for brown leather make up some ordinary tea (black with no milk). Leave it to brew until it goes cold, use it to stain brown leather then seal with leather conditioner.

For black leather you can try vinegaroon here