let from;

​

function setup() {

  createCanvas(400,400);

  to = color(0, 255, 0);

  from = color(255, 0, 0);

}

​

function draw() {

  for (let x = 0; x < width; x = x + 1) {

    let ratio = map(x, 0, width, 0, 1);

    let intercolor = lerpColor(from, to, ratio);

    stroke(intercolor);

    line(x, 0, x, height);

  }

}

​