Processing concept with a concept to draw on.

drawTest.pde

//https://processing.org/reference/clear_.html

// we use a canvas (PGraphics) to draw on. 
// in this case we can erase it (now with keypress)
// and we can display our "pen" without that it draws itself on the canvas.
 

PGraphics canvas;

color selectedColor;

void setup() {
  size(640, 360);
  //background(102);
  
  // make a smaller canvas then your window
  // in this case 100 pixels less 
  canvas = createGraphics(width-100,height-100);
  canvas.beginDraw();
    canvas.background(255);
  canvas.endDraw();
}

void draw() {
  
  background(0);
  
  drawSwatches();
  
  // we are in the menu (top)
  if(mouseY < 50) {
    // sample the color
    // and make the ellipse that color
    selectedColor = get(mouseX, mouseY);
    fill(selectedColor);
  }
  
  ellipse(mouseX,mouseY,10,10);
  
  canvas.beginDraw();
    if(mousePressed) {
      canvas.stroke(selectedColor);
      canvas.line(mouseX-50, mouseY-50, pmouseX-50, pmouseY-50);
    }
  canvas.endDraw();
  
  image(canvas,50,50);
  
}

void drawSwatches() {
  
  fill(255,0,0);
  rect(0,0,50,50);
  
  fill(0,255,0);
  rect(50,0,50,50);
  
  fill(0,0,255);
  rect(100,0,50,50);
  
}

void keyPressed() {
   // clean the canvas
   canvas.beginDraw();
   canvas.clear();
   canvas.endDraw();
}