If you want it to be colorfast, *do not* get Rit dye! It *will* fade, and after only about a dozen washes it will be gone. You want a Procion fiber-reactive dye that actually becomes part of the fabric.
Dharma Trading Company is a good online source, or if you have a local artists supply store, they may have Jaquard dyes. I have t-shirts dyed 7 years ago with Dharma dyes that have only now started to show signs of fading.