Take a plain orange T-shirt, and some iron-on T-shirt papers that can go through your printer. Go to your word processor Type in, "This is my costume. Scary, huh?" Make sure it's very big, and fits onto one paper. Follow the directions for the iron-on paper, and make sure the text is black (you will have to make a mirror image of your text, so that it shows up correctly on the T-shirt). Print out, then iron onto the front of your shirt, and there you have it, an instant 'non-costume'.
If you don't know where to get iron-on T-shirt papers, you can instead get black T-shirt paints and write 'This is my costume. Scary, huh?' directly onto the front.
Thanks to Teggo for this very unique idea!