I am wondering if Oxides are natural colorants? I have a Red Brick Oxide that does not appear to fade at all. I made soap with it about 3 years ago, I think and it's as dark now as it ever was. I do have to admit that I don't store this soap in sunlight, though and that I did use more of the colorant than I should because it really does bleed a lot of pink into the lather when used.
Oxides are not my colorant of choice, so I only have 3 that I bought when I attended a soap gathering and another soaper was de-stashing supplies. They are from reputable soap suppliers, but without looking, I can't say which supplier for sure.
As for micas fading, some do and some don't. I have a soap at sink-side, which gets sunlight through my kitchen window that is still as bright as the day I made them in Feb 2017. I used some Micas & More really bright colors in that soap, in fact, they are almost psychedelic even still. The only reason I put that bar of soap there is because the colors match the soap dish and the decor in my kitchen! It's been in this location for several months with no visible fading.