Tombenpotter suggests hooking into the render event and using GL11.color to apply a blue overlay to the mobs in effect.

Chill: combines the effects of slow and taking increased damage from all sources.

Frozen: cripples the mob's mobility entirely and take increased damage from all sources.

Chill will basically work on stacks, with stack 5 being frozen. Certain mod items will apply stacks faster depending on their level, and some will only go up to a certain level.

I've decided to just completely not do this.