Color of the experience bar while having spawn eggs, enchantment bottles, enchanted books and nether stars in the inventory

When you are in Survivalmode/Creativemode and you put a spawnegg/enchantment bottle/enchanted book/nether star in your hotkeybar, the color of the experiencebar will change the color. You must have an empty slot in the bar.
If the Item is for example on slot 5, there has to be another item on slot 1-4. Otherwise you cant reproduce it.