If you have a retextured item in your hand the xp-bar will change color

If you hold one of the following items

• any spawn egg
• enchanted books
• golden enchanted apple
• nether star
• grass, leaves any of these

the next thing happens:
you have an empty inventory -> take 1 of the items
your xp-bar will change color depending on which item you had.

if you add 1 of the same item the xp bar will turn back to normal having it green.

Check the screenshots for black and red xp-bars.