Incorporating ControlP5

I want to add the CP5 button into my state machine but I don't know how to remove the mousePressed function and instead make the button change the states. Please help!

import controlP5.*;

ControlP5 cp5;

int state = 0;

void setup() {

 size(600, 600);
 background(0);

 cp5 = new ControlP5(this);

  cp5.addButton("colorA");
}


void draw() {

 //state machine
 if (state == 0) {
   noStroke();
   fill(0, 255, 0);
   rect(0, 0, 600, 600);


 }


 if (state == 1) {
   noStroke();
   fill(255, 0, 0);
   rect(0, 0, 600, 600);

 }


 if (state == 2) {
   noStroke();
   fill(0, 0, 255);
   rect(0, 0, 600, 600);
 }


 if (state == 3) {
   noStroke();
   fill(128, 3, 255);
   rect(0, 0, 600, 600);
 }


 if (state == 4) {
   noStroke();
   fill(3, 249, 255);
   rect(0, 0, 600, 600);
 }


 if (state == 5) {
   noStroke();
   fill(254, 255, 3);
   rect(0, 0, 600, 600);

 }
}


void mousePressed() {
 state ++;

 if (state > 5) state = 1;

}