A cursor moving randomly around the screen examines colours of nearby pixels and places either border or body coloured pixels, changing palettes once the cursor hits the edge of the screen.

Cartridge code (276 chars):

u={1,-1,0,0}x=64y=64s=pset::★::x=mid(0,x+rnd(2)-1,127)y=mid(0,y+rnd(2)-1,127) b=11 c=15
d=3 e=.2
if (t()%1>.5) b=12 c=13 d=1 e=.05
s(x,y,b)a=pget(x,y) if ((a==b or a==d) and rnd(1)>e) s(x,y,d)
for i=1,4 do
f=y+u[5-i]a=pget(x+u[i],f)
if (a!=b and a!=d) s(x+u[i],f,c)
end
goto ★