void setup() {
  size(893, 50);
}

void draw() {
  background(204);


noStroke();
float r = mouseX;
for (int y = 0 ; y < 50; y += 10) {
  for (int x = 0 ; x < 893; x += 10) {
    fill((x+y) * map(r, 0, 893, 0, 3));
    rect(x, y, 10, 10);
  }
}
}