//if2.pde
void setup() {
  size( 500, 500 );
  smooth();
}

void draw() {

  if ( mouseX < 100 ) 
  {
    background( 255, 0, 0 );
  } 
  else if ( mouseX < 200 ) 
  {
    background( 0, 0, 255 );
  } 
  else {
    background(0, 255, 0);
  }
}