createCanvas(800, 800);

  background(220);

  fill(255,0,0);

  rect(0,0,50,50);

  fill(255);

  rect(0,50,50,50);

  fill(0,0,255)

  rect(0,100,50,50);

}

​

var paint = "white";

​

function drawLine(mx, my, pmx, pmy) {

  line(mx, my, pmx, pmy);

}

​

function draw() {  

  if(mouseIsPressed && mouseX<=50 && mouseY <=50){

    paint = "red";

    print("Changed to Red");

  }

  if(mouseIsPressed && mouseX<=50 &&mouseY >=50 && mouseY<=100 ){

    paint = "white";

    print("Changed to White");

  }

  if(mouseIsPressed && mouseX<=50 &&mouseY >=100 && mouseY<=150 ){

    paint = "blue";

    print("Changed to Blue");

  }

  if(mouseIsPressed && paint === "red"){

    stroke(255,0,0)

    strokeWeight(3)

    drawLine(mouseX, mouseY, pmouseX, pmouseY);

  }

  else if(mouseIsPressed && paint === "white"){

    stroke(255)

    strokeWeight(3)

    drawLine(mouseX, mouseY, pmouseX, pmouseY);

  }

  else if(mouseIsPressed && paint === "blue"){

    stroke(0,0,255)

    strokeWeight(3)

    drawLine(mouseX, mouseY, pmouseX, pmouseY);

  }

}