Ok... I've done about 7 sets of Manic Panic Hair.... I swear by the stuff, but you HAVE to bleach it out... HAVE TO.
When I went pink, I had to go to platinum so that the vibrancy would show, AND because lifted hair holds semi-permanent color longer. It's because when you bleach it, the pores are more open and receptive to more color.
My advice. Get a bleach kit from Sally's or somewhere like that. Pin up the hair you don't want to dye and just dye the hair you plan on coloring. if you don't lift your hair beyond the orange/ strawberry looking shades, your hair WILL turn out green. (Had this experience with my husband when he went to a concert once).
Apply the Manic Panic to the lifted hair, put on a cap and HEAT IT. You can do this by either sitting under a dryer, or by using a hand held dryer. You have to keep it covered though, because you want to heat the hair, not dry the dye.
After you do this, rinse it out. If you see that the color is fading or you want to refresh the color, all you have to do is add a dime sized amound to your conditioner, run it through your hair and leave on for the duration for your shower.
Since you are putting the color underneath, you really don't have to worry about touching up the roots. I had to touch mine up about every three weeks because it was all over color, and black roots with pink hair really stand out.
if you have any other questions, please feel free to message me. I've had almost every experience with Manic Panic you can think of