Sensible!
... Looking at the Element Res and Amp numbers again. Green is Good, and Red is Bad, right? It seems like the Elemental Amp coloring is acting like it's Elemental Res, sort of.
It might be easier if both Res and Amp had their base values be 100%? A.k.a Basic Brill deals 100% Elemental Damage, and Dullstone makes her deal 67% of it?
Otherwise:
1. Wearing just the Dullstone shows "+-33%" Elemental Amp as Green.
2. 100% Elemental Amp is neutral (White?)
3. >100% Elemental Amp is Red