createCanvas(200, 300);

  textSize(32);

  fill(255);

}

​

function draw() {

  let xPos = constrain(mouseX, 0, width);

  let redValue = map(xPos, 0, width, 0, 255);

  let yPos = constrain(mouseY, 0, height);

  let blueValue = map(yPos, 0, height, 0, 255);

  background(redValue, 0, blueValue);

  text(`red: ${redValue}\nblue: ${blueValue}`, 10, 40);

}