let rectWidth;

​

function setup() {

  createCanvas(400, 400);

  noStroke();

}

​

function draw() {

  //numRectangles = map(mouseX, 0, width, 4, 20);

  rectWidth = width / numRectangles;

  background(150, 120, 182);

  for (let i = 0; i < numRectangles; i++) {

    let posX = rectWidth*i;

    let posY = height - rectWidth*(i+ 1);

    let rectHeight = rectWidth*(i+1);

    let fillColor = map(i, 0, numRectangles - 1, 0, 255);

    fill(fillColor);

    rect(posX, posY, rectWidth, rectHeight);

  }

}