If you put it through the dryer without noticing, probably nothing as it is set. If not, I would put it through a hot cycle with bleach.
Find a Rit Dye display and look for the Color Remover, which is made to take color out of white fabrics that have accidentally been washed with colors. Walmart carries Rit Dye, Target or Joann Fabrics might, and I know I have seen it in several (Publix) grocery stores. It might take a couple washings in the Color Remover to completely get it all out but it is more gentle on your fabric than repeatedly washing with the shirt with bleach.
Thanks, I forgot I had some of the Rit Dye Color Remover - it worked! Thanks for the help.
Yay! So glad it worked for you. We all have those "oops" laundry moments sooner or later (and sometimes repeatedly!) so I should follow my own advice and pick up some Color Remover to keep on hand. :)