I've identified and issue 'how much health you will lose from a DOT effect' colour change to health bars. Except in the case of true damage, the amount 'greyed out' as it were will change based on the targets resistances, whether or not he is shielded or has certain temporary buffs like Garen's courage or Sona's W active.
If you show the entirety of 'this is the amount they are losing from this effect', those things make the health bar a busier place, and if it just shows 'this is how much you lose per tick' it isn't really worth it due to the fast refresh time.
And then there are the cases that you can't really estimate well like Morgana's Tormented Soil or the sudden aplification of damage from Liandrys if you happen to walk through the enemy Lux's lucent singularity.
On one hand it helps, but there are a few disadvantages as well.