Hoping To Go Back To Natural Color

If you choose to do a DIY treatment, invest in a protein filler. Bleached
hair is sooooo porous (I'm assuming your hair is bleached if it's platinum)
that it will soak up all the color and do it unevenly too. So use a protein
filler before coloring and it will fill the pores. ALSO with hair this light,
get a shade two shades lighter than what you want or else you will end up
with a beautiful espresso, but probably not what you're going for...

Or you can just pay the bucks for a salon treatment, but my philosophy is
as long as you're careful and well-educated before hand, there's nothing
they can do at a salon that you can't do just as well.

Whether its bleached or not, if its white you may have a problem coloring it in that the base of the dye usually shows up.  Say you choose a dye with a violet base, you may get gray or purplish hair.  If you choose one with an orange base, it may come out too orange, likewise with any color.  To fix this problem when I went from white to darker, I dyed it twice and used opposing bases (one brown with orange base, one brown with green base) to give me a nice brown...came out a reddish brown, but that was better than greeny brown). Since you only want to go light ash blonde, I'd suggest getting a dark blonde with a gold base and a violet based blonde so your yellow and violet cancel each other.  If you want to be more ash, use the violet based one last. 

Let me tell you though, I'm not a pro and I never tried it with gold and violet.  In theory it should work, but I only used red and green to go brown on my own self.  Since it IS kind of complicated, a salon might be easier.  I have also not tried protein filler, so I don't know if that would fix the base problem. I know it helps fill the porous hair, and is what you should use in that respect at helping your dye come out better, but I suspect it wouldn't fix the base problem since your hair is still white and has no pigment left to counter the base color in the dye (so your hair can easily come out the base color instead of the color its supposed to).
 

 

Really?  That's good to know, thanks!