Mark the heathens you call humanity, while maintaining their ignorance. Are you born of the higher order, or do you wish to mark your own, in a pitiful effort to leave the curse of the muggle-born. Here is a simple elixir to permanently mark the robes of the lesser kind.
Step 1: Materials
1. A Trusty Wand (A black-light. Mine is a pen with a small black-light LED)
2. Robes To Mark Upon (Colors White to Brown Should Work)
3. Muggle Board (Cardboard To Separate The Elixir From Direct Eyesight)
4. Lumos Maxima (Highlighter)
5. Obliviate (Optional: A sharpie the same color as the shirt, to make corrections.)
Step 2: Practice, and Setup
Scribe muggle a few times on your piece of muggle board because your infantile dexterity lacks the capacity for perfection.
Slide the piece of muggle board into the T-Shirt, parralelous, sorry, parallel to the bottom of the shirt.
You are now ready to make your mark. Gosh, that took ages!
Step 3: The Spell
This is meant to be a very delicate technique, even an aspiring wizard isn't expected to master it within his first year at Hogwarts. It is like giving a cow a club, and expecting him to make a hole in one. Guh... I guess I may as well try.
Simply say, "lumos maxima!", and scribe the desired phrase "Muggle" across the shirt, centered upon the opening of the neck. Make sure to scribe above the muggle board, in order to prevent all hell from breaking loose.
Repeat this process as many times as you wish, and don't muck it up!
Step 4: The Result
The result is impossible to capture on your worldly machines in all honesty. Luckily, your eyes are trained to the extent of an aspiring wizard. What I would see in broad daylight with a simple spell, you can see only in complete darkness, under the light of your trusty wand. I have merely replicated an after-image to register on your arcane devices.