Good point Wazz. Mordants are more about fixing dye so it won't wash out etc.
As for colour retention vs oxidisation, that is a question we will have to wait for the answer for..
Before I took to Luthiery full time, I was a Stonemason.
I had a restoration job on a heritage building in Sydney and the mortar was a pinkish brown.
Never saw that before in my life, I initially thought they had used a pink sand but on closer inspection I saw it wasn't the sand..
Did my research (asked an old timer Stony) and discovered the mortar was probably stained with Ox blood!!!
So off I go to the local slaughter yard and got a 20litre drum to run some tests.
When fresh the mortar was pink, but over a few weeks it changed to the pinkish brown due to oxidisation so your question about colour fastness is a beauty![]()