Mob heads turn white when you hit them

When I summon a mob, skeleton to be exact, with /summon, I add a player head in the head slot. I used /summon Skeleton ~ ~1 ~ {Equipment:[{},{},{},{},{id:397,Damage:3,tag:{SkullOwner:"MHF_Pig"}}]} in a command block. The problem is, if I hit it, the head momentarily turns white.