Thank you for your comments !!
Wow that is a tuff one I have never applied any type of coloring to Antron other than the known method of dying the material. I have heard from knowledgable people that marker sticks very well to Antron but I have never personally done it. I suggest grabbing a piece of spare Antron fabric the same color as your muppet and doing a test. Let me know what happens, I would love to see pictures of your muppet!