I just came across this, rather interesting. It's certainly put me off Angelus leather dyes (which is a shame, because they can be found quite affordably, have lots of colours and look great when Derek from BigD guitars puts them on)
This guy put some samples of four different dye brands on to a test piece and then stuck it in his window so it would get the full sun every day, and these are the results after 20 days. Admittedly it's not really a 1 for 1 comparison, he just used the colours he had in his shop.
It seems that a week later (not shown in the pics) the Angelus is even worse.
What's also frustrating is that the two brands that did well (Keda and Colortone) don't seem to be readily available here. There are probably others which are as colourfast, but without testing who knows?
Colortone is available from Stewmac - sometimes they refuse to ship it but lately seems OK.
I actually meant to write Transtint rather than Colortone, but it seems they are the same product anyway but marketed under different brands.
It's a shame the Colortone is only available in those 2oz bottles, going by how concentrated that stuff is I think those of us who just want to make a few guitars would never even use one up.
Being able to get a full selection of the colours in much smaller bottles (a la Crimson Stunning Stains Shots) would be much better for the occasional user IMO.
Light colours, especially greys and blues are notoriously bad for fading. If you use one of these then try to make sure you have a UV rated lacquer to protect the stain underneath. I think this can also be boosted with an additive but you would need to speak to your lacquer supplier to find out how much UV protection any particular lacquer will give.