There really aren't any good temporary dyes, not even Manic Panic. I dyed my hair magenta last year (let's call it an experimental phase XD) and it was a pain. That stuff rubbed off on all of my clothes (ruining some) and all over my skin. I even noticed a couple days after that my white phone had turned pink and I could NOT get that stuff off.
So yeah, for the sake of not ruining things you own and having parts of your skin turn gray for awhile, I would just buy a wig. If you're concerned about arrival time, ordering from US sellers like Arda and Epic Cosplay will get you a wig within days. And if you ever want to do the cosplay again, you won't have to screw up your hair/clothes/skin/belongings every time.
(Also, I love that you're cosplaying from a Slender-series!)
Conventions for 2015 (so far...): Momocon, A-Kon, Otakuthon, Sac Anime
Cosplay line-up: Mayoi Hachikuji (Bakemonogatari), Nona (Death Parade), Nagisa Momoe (Madoka Magica), Menma (AnoHana), Tsukihi Araragi (Bakemonogatari)