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!)
ACen 2014 cosplay: Lara Croft, Erio Touwa, Shirahoshi Hime
ALA 2015 cosplay: Shirahoshi Hime, Akemi Homura [Haregi ver.], Erio Touwa