My experience with Rit dye has NOT been good...I would go to the craft dept or a craft store and buy the tie dye kit (or individual packages) that have soda ash with them...they tend to provide a more vibrant, even color and do not tend to bleed or fade as bad as Rit...I have tie dyed dozens of shirts with my two Girl Scout troops and we have had wonderful results with the dye with soda ash...I am with some of the previous posters though, I would bleach the entire shirt and start over...you will never match the exact color.
Good luck!!