Done with an IF/ELSE statement

Super Stunning Psychedelic Booleans v0.2.pde

/*
Steve Lambert 
Super Stunning Psychedelic Booleans
September 15, 2015 v0.2
Done with an IF/ELSE statement
*/

void setup(){
 size(800,600); // how big it is, you know this.
 println("Welcome to Steve's Psychedelic Light Show");
 println("Width: " + width); // dumb question, dumb answer
} // end setup

void draw(){
  background(250); // dull
  
  // THIS WORKS
  if (mouseX <= width/4 ) { // if we go to the left quarter
  // then do this:
        background(255,0,0); // paint the town red
        println("X: " + mouseX);
        } 
        
        else /* otherwise... */ if (mouseX >= (width/4 * 3)) {
        background(0,0,255); // paint the town blue
        }
        /* if none of the above is true, forget I mentioned it at all and go back to what you were doing */

  
} // end draw