Use the mouse clicked method. Look at the reference (Input-mouse section) for other mouse event methods

1. void setup() {
2. }
3. 
   
4. void draw() {
5.  
6. }
7. 
   
8. void mouseClicked(){
9.     println("mouse was clicked");
10. }

The code below shows how you can control what is drawn by clciking the mouse.

Simply changing the value in state changes what is displayed. The code in the mouseClicked event moves to the next state but you could use a switch statement to perform different things for each state (use switch statment like in draw)

1. int state = 0;
2. 
   
3. void setup() {
4.   size(300, 300);
5. }
6. 
   
7. void draw() {
8.   switch(state) {
9.   case 0:
10.     drawA();
11.     break;
12.   case 1:
13.     drawB();
14.     break;
15.   case 2:
16.     drawC();
17.     break;
18.   case 3:
19.     drawD();
20.     break;
21.   }
22. }
23. 
    
24. void drawA() {
25.   background(255); // White
26. }
27. 
    
28. void drawB() {
29.   background(255, 255, 60); // Yellow
30. }
31. 
    
32. void drawC() {
33.   background(60, 255, 60);
34. }
35. 
    
36. void drawD() {
37.   background(60, 60, 255);
38. }
39. 
    
40. void mouseClicked() {
41.   state++; // add one for next state
42.   if (state > 3)
43.     state = 0; // limit state
44. }

the code inside mouseClicked is executed everywhere in the sketch.

I think you mean changes to variables made in mouseClicked are visible everywhere in the sketch code. That is true but it is also true for the draw() method as well.

Ah. You need to make the code inside mousePressed() conditional on what was viable when the user clicked. You can do this by tracking the state of your program, and updating the state when each click is done.

1. int state = 0;
2. 
   
3. void setup(){
4.   size(220,220);
5. }
6. 
   
7. void draw(){
8.   if( state == 0 ) background(0);
9.   if( state == 1 ) background(255,0,0);
10.   if( state == 2 ) background(0,255,0);
11.   if( state == 3 ) background(0,0,255);
12. }
13. 
    
14. void mousePressed(){
15.   if( state == 0 ){ state = 1; return; }
16.   if( state == 1 ){ state = 3; return; }
17.   if( state == 2 ){ state = 0; return; }
18.   if( state == 3 ){ state = 2; return; }
19. }

You guys are amazing! I solved the problem, finally! :)