LOL! I did a make-up test of Ryuk
last night. Here's what I found out.
If you can, try an air-brush. My grey make-up was actually grey and white Halloween creme make-up mixed, and it looked streaky. In some places, I had to apply it really thick.
For the black around the eyes and mouth, liquid eyeliner works OK, but it smells and tends to peel after a while. The black lipstick I used didn't cover very well, and my lips felt funny with it on (I don't think it was an allergy, but I'm going to be cautious if I use that stuff again). I ended up going over both the eyes and mouth with black creme make-up again -- make sure that what you use is non-toxic and can be used that way.
For the eyes, I might cut a pingpong ball in half and have fabric pupils so I can see. The nose will be the hardest part; I'm still not sure how that'll work yet.
Hope that helps!