I use acrylic paint, which washes out but usually fades to a vintage-y look after awhile. I'll probably update this later with more photos on how the uh t-shirt wash cycle progresses.
Anyway. Biology-inspired tee, complete with a tonoplast, nucleus, golgi, endoplastic reticulum and a variety of lively plastids. Appropriately nerdy.
Oh, and sorry about the foul language, there.