let R = 100;              // R is red in RGB for changing color

let originX = 200;        

let originY = 200;

let colorCoe = 1;        //  color coefficient 

​

function setup() {

  createCanvas(originX * 2, originY * 2);

  angleMode(DEGREES);

}

​

function draw() {

  background(220);

  stroke("black");

  strokeWeight(2);

  fill(R, 255, 255);

  translate(originX, originY);    // move the origin to the center 

  rotate(angleRotate);            // let the shape rotate 

  rect(0,0,100,100);              // the shape rotates around the origin

  angleRotate += 1;               // increase a degree after every loop

  R += colorCoe*1;      

  // the colorCoe will change when R reaches 255 or 100

  if (R === 255 || R === 100) colorCoe *= -1;

}