Can be either wall or ceiling, depending on what you like. Could even
be different if you like it.

If it were me, I would let the slope and knee wall height determine. If
the knee wall were taller than about 5 or 6 feet then i would do
ceiling. I would also say that if the slope were more perpendicular
than horizontal I would paint it wall colored.

So, if it was a toss up on slope I would let the height determine which
I would use. If its a low knee wall I would use wall paint, if a tall
knee wall I would use ceiling.

Clear as mud huh?