let spacing = 20;

let circles = 15;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  for (let i=0; i<circles; i++){

    fill(255*(i/(circles-1)), 20, 120);

    ellipse(x0 + i*spacing, y0 + i*spacing, s0 + i*10, s0 + i*10);

  }

}