So I prepared another test to compare Keda dyes with some Colortone to see if any dyes would have the same issue.
Only leftover colortone I had is red, so I mixed a red Keda dyes mix and repeated the test, with Keda and Colortone.
Not the same tint as I didn't really try to match them, but both pure red, and as before I kept a reference in the dark in my garage while I exposed the other part.
Before (Keda on the left, colortone on the right):
And after a month (Keda at the top, and colortone at the bottom):
They both lost some colour, but the Colortone resisted a lot better than the Keda dyes...
So Keda is out...
Now, I still want to taint that guitar purple / magenta / violet... so I was looking at getting some colortone from Stewmac, but I was looking at 3x bottles (black, blue, red) and that is a bit steep as I'm not planning too many more blue / red guitars...
And just then Stan posted amazing photos of his purple guitar: http://www.buildyourownguitar.com.au...l=1#post123864
So I just went to get some artist ink, and I'm going to do more testing... but I'm done with the lightfast testing, I'll only test mixing the 3x colours I got blue, violet and black