Heya,

Hmm, I notice that some cels will add "backing
colors." Namely, I've seen cels painted
all black or have some parts painted white over
the back where it's painted.

This is usually to prevent the effects of the
background from being seen through the cel.

For flesh tones, I actually recommend mixing
a few drops of green in the paint.
I used to do this all the time when I was
a kid and I found that it made flesh look more
"toned down" and earthy.

Then I went to college and found out that Leonardo
Di Vinci did the same thing! O_o

Use a small brush, thoroghly wet it first
and maybe do thinner layers to prevent the
bubbles.