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!)
Fandom Fest 2014 cosplay: GLaDOS (Portal), Ellie (The Last of Us), Lara Croft (Tomb Raider 2013)
MCM London Comic Expo cosplay: Oktavia von Seckendorff (Madoka Magica), Fem!Slenderman,
Future cosplay: Faith Connors (Mirror's Edge), Lara Croft (Rise of the Tomb Raider), Tetra (The Legend of Zelda: Wind Waker)
2015 conventions: Anime Los Angeles, Anime Central, Japan Expo