There is already software that simulates
a "Paint Ball" game. (People shoot each other
with paint pellets; colorful splotches tell you
you've hit someone.)
In the multiuser graphics game I'm thinking of,
the paint you shoot is "intelligent"; you can
supply a function for it, not just a single
and paint comes in buckets, not just single
Users have a limited (but large) amount of
paint that is keyed to them, performs their
computation. (This is a nice metaphor because
the computation available is in fact limited.)
There are a couple of ready-made filters
to choose from, and there is a way of
trading new filters.
The one with the most paint worn by
other players wins. Users can
traps that spray paint on other users lured
into them, or attack others with painting
weapons, or make paint that is so
attractive that people voluntarily use it.
Paint wears off over time (or maybe can be
washed off), but is layered; in particular, paint
can be an effect on the layer below it.
(Taken to the next level, paint could be a
transformation on the underlying 3D model
of the user's avatar; shooting people could
grow spikes on them, or make them smaller.)
There could be Spy Paint that reports its
surrounding image to its owner; the physics
of the game would have to be accurate enough
to make sure that a little drop sees less than
a whole bucket.