Late arrival to this party, sorry. As you may of seen in my thread (thanks for finding it GMB) I tackled this a couple of years ago now, covering my black seats with a very light grey.
Most of my findings and thoughts are in my thread. There's NO WAY I'd do it again or even recommend it. After about a week it just flakes off and ends up colouring your clothes more than the seats.
Sorry, I couldn't resist. My seats still look great two years on. My main issue is the same that anyone would have with a light coloured leather seat from the factory. They can look a little dirty if not kept on top of - but this is just leather maintenance and conditioning that should be done anyway. The colourant did come off slightly revealing the black on a couple of areas that your clothing rubs against whenever you get in/out of the car, but this is very easily touched up with a paintbrush and doesn't need touching up again for a good few months, maybe even longer depending on how much you use the car.
My first attempt was with a sponge and that looked awful I'd definitely recommend investing in a half decent airbrush to apply the colour and sealant, it gives a finish that is 100 times better than applying with the sponge.
I'll take some pics of my seats at the weekend so you can see how a light colour has 'weathered' after two years use. It certainly hasn't just rubbed off like some thought it would.
Take your time and do it right first time (unlike I did)