Thanks everyone! I know that this response is really late. I didn't get to see any of these responses before the convention, so I just wound up using sharpie in the end. These will be great for future reference, though!
The red & black sharpie DID rub off onto the inside of my shirt a little bit, but not enough to show through the front or anything (thank goodness).
I wanted to wash what's there out, but I'm worried that it will just bleed through the front if I try. Any suggestions for what to use for this, or should I just leave it alone since it can't be seen on the outside how it is now?
Anyway, I'll definitely be putting your suggestions to use the next time I wear this costume. Thanks again!