Manipulate[
 t1 = Triangle[{{0, 0}, {x, y}, {1, 0}}];
 t2 = Triangle[{{0, 1}, {x, y}, {1, 1}}];
 t3 = Triangle[{{0, 0}, {x, y}, {0, 1}}];
 t4 = Triangle[{{1, 0}, {x, y}, {1, 1}}];
 Graphics[{Blue, t1, Red, t2, Green, t3, Yellow, t4},
  ImageSize -> 300],
 {x, 0, 1}, {y, 0, 1}]

{DirichletCondition[{f[x, y] == 0.}, y == 0],
 DirichletCondition[{f[x, y] == 1.}, y == 1]}

NeumannValue[0, x =0||x==1]