Strange Armor GUI bug

If you look at the Armor menu, and get hurt while wearing armor, the armor will surround the body part in a strange colored block.

What I expected to happen was:
Nothing when I got hurt while looking at the Armor menu and wearing armor.

What actually happened was:
For me, I was only wearing a simple leather cap as armor and when I got hurt by a zombie while looking at the armor menu, Steve's head was surrounded in a strange translucent red block and the slot for the "head armor" was colored in dark red.

Steps to Reproduce:
1. Put on armor
2. Let a hostile mob approach you
3. Go to the armor menu before you get hurt by the mob
4. Look at Steve when he is hurt.