function setup() {
  createCanvas(400, 400);
  background(220);

  var x = 10;

  // rect(10, 10, 1, 380);
  var color_division = 255 / 780;
  var i = 0;
  while (i < 780) {
    const c = color(
      0 + color_division * i,
      0 + color_division * i,
      0 + color_division * i,
    );
    fill(c);
    noStroke();

    rect(10 + i, 10, 1, 380);
    i += 1;
  }
}

function draw() {}