Agreed Jimi! Been using VT VG-nic for a couple weeks now. Just outstanding on all levels.
But I don't think yellow/brown is plant material. It might be, but I think it is also oxides, and probably not just nic oxides. Probably oxides of other alkaloids present in lesser pure nics. My VT was a slight pink too. I really doubt it is anything harmful.
Your statement about pink being from purer nics slightly oxidizing sounds reasonable, but do you have any literature to support that? I have never found anything online that explained the pink rigorously. I have been just going by what I thought was happening. Previous to VT, the only pink nic I had heard of was Xtreme
vaping (Levy), which of course is very pure as well.