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!)
Tsumugi Kotobuki - Waitress Ver. [ K-ON! ] - 100%
Yuki Nagato - Disappearance Ver. [ The Disappearance of Haruhi Suzumiya ] - 100%
Lara Croft - Reboot Ver. [ Tomb Raider ] - 90%
Kyouko Sakura - Casual Ver. [ Puella Magi Madoka Magica ] - 100%
Makoto Sawatari [ Kanon ] - 100%
Charlotte - Human Ver. [ Puella Magi Madoka Magica ] - 40%
Ayu Tsukimiya [ Kanon ] - 10%