vertex(1). vertex(2). vertex(3).
edge(1,2). edge(2,3). edge(3,1).

color(r).  color(b).  color(g).

colored(V,C)    :- not othercolor(V,C),
                   vertex(V),color(C).
othercolor(V,C) :- colored(V,C1), C != C1,
                   vertex(V),color(C),color(C1).
                :- edge(V,U),color(C),
                   colored(V,C),colored(U,C).

#hide .
#show colored/2.