I used the Gliptone and it did work out well - also in the Buckskin colour they had on file (they had two actually, this was the earlier one and the match is perfect). I literally had no colour left only the brown/grey of the raw hide. In other words the leather was knackered. It worked well. If I was to share my experience it would be as follows:
1. If you can, avoid doing this at all because even if as in my case the results are good, the feel isn't really the same as before. If you have genuinely good items of one colour, try and trade them if you can because you are effectively destroying them. For "honestly worn" items, learn to love the patina rather than trying to make them as new. For faded rather than worn items, there are alternative ways of dying directly into the leather to bring back colour rather than the techniques here which effectively build up a new layer on the "connolization". The old fashioned "Feiblings" does the job here. Just don't wear a white linen suit for a few months ;-)
2. Changing colour light-dark is far easier than dark to light. Yeah, should go without saying for anyone who has ever painted anything but it is true. If you are going lighter with leather, then you'll need to strip the dark colour off with cellulose thinners first. With vinyl if you are trying to go lighter, you will need so much of the product, you'll lose some of the gain effect.
3. The same product won't work for leather and vinyl. The vinyl product permeates the surface, the leather is a new "skin". This unfortunately does not allow the leather to "breath" as before and you can't condition it easily in future.
4. By far the best way to apply the leather colour is by airbrush. I used mine with an ordinary air compressor. Build it up in light coats. It takes patience as coverage is terrible to start with. Took me about 10 coats, the result is flawless where the leather was good and prepared properly. You can speed drying up a little with a hairdryer but the process done this way will take several days. At the end I left it for a week to dry thoroughly. A blow gun is also useful for cleaning and blowing the dust you create sanding.
5. Seams are really awkward, tape them back to allow the colour in, otherwise you will get voids and shadows. They are also vinyl even on leather seats, so do these first with the right product - the leather one won't stick.
6. Condition and clean thoroughly. Ideally treat the leather weeks or months in advance. It will be more difficult to condition seats in future so do this well. This is really important.
7. Ambla as in the P6 has a very distinctive effect on the grain as in it looks "dirty" (you know what I mean by this). You will lose this which may or may not be desirable, the finish will be uniform and "clean". With the exception of black, even if the colour match is perfect, it won't look original. Nothing you can do about this. It does however make door panels and the door pull match properly, which is nice - mango owners take note...
8. If you use a light colour (like buckskin), avoid wearing jeans, except really old or bleached ones as you will get dye transfer.
9. For leather filling buy the Furniture clinic product: It actually comes in a sensible size for not much money. Forget the silly Gliptone little pots.
10. Really concentrate on softening the creases and cracks and then fill these. You'll find many creases simply disappear and don't need work - work conditioner into the leather with you hands. With cracks, soften the edges with sandpaper a little BEFORE filling. You can really work miracles here. You need a lot of patience and work on sanding the leather surface even compared to preparing panels for painting.
11. Like panel work your results will depend on how much effort you put in. Practice first on say the rear arm rest until you get the feel for it.
12. I found no matter how much I cleaned some items I got areas in which the product wouldn't stick very well at all. Obviously in the last 45 years I have no idea what potions were used to clean or what nasties were spilled on it; there were definitely dog-based remnants in mine....
13. Order more than you need of the vinyl product as you will get scuffs to touch-up. The leather colour has a shelf life of 6 months apparently so you can't do this for that.
In conclusion, for me this is only a stop gap and I will get them recovered. The colour kits are great products but I have very mixed feelings about them. I kind of like an honestly aged car to one that tries to be perfect. Like I said, my situation was the seats were nearly fit for the dump, as such I'm pleased they are now serviceable and almost "good" except on close inspection. I would recommend this for my situation of extreme wear and also as in a new car for touching in scuffs, marks etc. There's a whole field of patina and wear between those two extremes I would probably not use this on, I would just deep clean and condition. I probably wouldn't choose to recolour any item unless I could not avoid it.