Very calmly and without any sudden movements...STEP AWAY FROM THE RIT!
It is just unreliable as far as color permanancy and fading go.
That being said you can definately overdye your shirt. You could do dye it Navy again or use another color that would compliment and blend well(depends on how faded it is) You should use fiber reactive dyes. A great source for dyes and instructions is at
Tie-dyed.com
Its real easy to do. You can do low-water immersion to get a solid color. Here are
instructions
You will need soda ash which is available at tie-dyed.com too or most pool supply places have it. Its called Ph UP.
Anyways...its real simple to do.
Oh by the way..there is a forum connected to tie-dyed.com. Join the forum and you get 10% off your purchases at tie-dyed.com. After you join email the administrator(Michael) and he will update your account to get the discount.
Kathy