size(400, 400);

for (int x=0; x < width; x++) {
  for (int y=0; y < height; y++) {
    float percentX = 1.0 * x / width;
    float percentY = 1.0 * y / height;
    stroke(255*percentX, 255*percentY, 255*(1-percentX)*(1-percentY));
    point(x, y);
  }
}