Ever used a rocket hammer charge against a weak enemy? The damage numbers aren't yellow like they're supposed to be.
If I recall correctly, this happened in a patch shortly after lost souls got introduced to avoid lost souls making huge damage numbers all the time. I went into Tier 3 Guild Training Hall to find out what the limit was. We found out this:
468 damage shows yellow numbers.
473 damage shows white.
Since T3 bags have 61 defence, the limit for yellow numbers become somewhere around the defence being 11.5% of the total damage inflicted.
Reducing enemy defence also reduces how much damage you have to inflict before the number becomes white again. At the moment, I'm too tired to go much deeper into it, but research involving poison and kat debuffs may follow.
I don't pay attention to the colors --- or even the numbers, unless I'm collecting data. But it's great that you're reverse-engineering this obscure aspect of the game. Please continue.