Love the yin/yang look but I think it's a bad idea because then you need 2 numbers. One showing the number of purify counters, the other showing the number of Neurotoxin counters. But I think this is a glitch. When you use purify, you shouldn't still have neurotoxin, so this suggestion wouldn't be needed.
Seems you are a bit confused.
When a player has a purity counter, like +6, and then they are hit by neurotoxin, their purity will decrease, but remain blue until it goes negative.
So, you might not recognize you have been hit with neurotoxin and continue to play extra pillars, or what-not, and harming yourself inadvertently. And then when it turns yellow, you are already in the negative.