​

function setup() {

  createCanvas(710, 400);

  dim = width / 2;

  background(255);

  colorMode(HSB, 360, 100, 100);

  noStroke();

  ellipseMode(RADIUS);

  drawGradient(width/2, height / 2);

}

​

function drawGradient(x, y) {

  let radius = dim / 2;

  let h = 0;

  let s = 80-0.5*radius;

  let b = 100;

  for (let r = radius; r > 0; --r) {

    let c = color(0, s, b);

    fill(c);

    ellipse(x, y, r, r);

    // Adjust saturation and brightness

    s += 0.5;

  }

}