Hi, what color was your hair before the Beyond the Zone?
Whatever you do, first you should use a strong cleansing shampoo (make sure to use a deep conditioner after) a few times to remove some of the color buildup. Paul Mitchell makes Shampoo 3 for this purpose. (And Joico K-Pak reconstruct is great, because your hair will get dried out.) Since you say they are only stains some of the color should wash out.
- wash with any shampoo containing green pigments. For some reason, the chemicals that compose the green colors can help to fade out muddy colors and dyes in general.
I have to say that that advice is very wrong. Green cancels out red tones in hair, brassiness. Green is a lot more drab on the color scale and would make muddy colors look even muddier. You actually probably need a high level (maybe a 8) RO or Red Orange to give your hair back some of the pigment that it has lost, before reapplying the purple.. I am guessing that you were bleached blonde before the purple was applied? To look beautiful and healthy hair has to have some degree of all three primary colors in it-- blue, red, and yellow. Grey is usually an effect of green, blue, or violet, so you most likely don't need anymore blue. To get a permanent vibrant purple (or even any natural shade of dark blonde-brown-black) you especially are going to need to get the gold and red tones back into your hair (this is what bleach pulls out as the hair lightens to yellow or white) first.
If you are just going to slap more purple on it I would suggest Inkworks because it is a vegetable dye and is non-harmful to the hair and washes out completely after a week or two of shampooing.
The part of all permanent hair color that makes it damaging is the developer, which blows open the cuticle so that the color can deposit into the hair shaft. Healthy hair is not compromised by this but hair that has been bleached, stripped, dried out, is already damaged and cannot handle the process as well.
I would wash it with the above for about a week, then use Paul Mitchell's PM shines 8 RO (for lighter) or 6 RO (for medium) depending on the level of your hair color now, as a filler, before applying Inkworks purple. Both of these products are completely non-damaging, as PM shines is a conditioning demi-permanent and Inkworks is a temporary color. Sally's does not sell PM though, you might order it online. Good luck.