let lastredd;

function setup() {

  createCanvas(600, 400);

  redd = false; // if it is red

  lastredd = false; // if it is red before

}

​

function draw() {

  background(220);

  // draw 2 rect

  fill(255);

  rect(200, 0, 200, 400);

  rect(400, 0, 200, 400);

  // set condition

  if (mouseX <= 200 && mouseX >= 0 && !lastredd) {  // if the mouse is inside of the box and it is red before

    redd = !redd; // no red fill

    lastredd = true; // it is red before

  } else if (mouseX > 200 || mouseX < 0) { // mouse outside of the first box

    lastredd = false; // reset state

  }

// link redd to fill red

  if (redd) {

    fill(255, 0, 0);

  } else {

    fill(255);

  }

​

  rect(0, 0, 200, 400);

}

​