Rectangle button

you can enhancement your code your start is good

here you can add buttons and control with mousePressed

Button red;  

void setup() {
  size (600, 650);
  smooth();
  background(0); 
  
 red = new Button("red color", 20, 20, 100, 50);
}

void draw() {
red.Draw();
  }

void mousePressed()
{
  if (red.mousePressed()) {
   
    fill(222,0,0);
   
    rect(12,111,230,230);
}

}

class Button {
  String label; 
  float x;     
  float y;      
  float w;      
  float h;      
  
  Button(String labelB, float xpos, float ypos, float widthB, float heightB) {
    label = labelB;
    x = xpos;
    y = ypos;
    w = widthB;
    h = heightB;
  }
  
  void Draw() {
 fill(222,0,0);
    stroke(141);
    rect(x, y, w, h, 10);
    textAlign(CENTER, CENTER);
    fill(0);
    text(label, x + (w / 2), y + (h / 2));
  }
  
  boolean mousePressed () {
    if (mouseX > x && mouseX < (x + w) && mouseY > y && mouseY < (y + h)) {
      return true;
    }
    return false;
  }

}