The if in mouseClicked() is only there to check which color you currently have and choose the other one (toggle).
BUT for a pure random, the if is not needed.
Just say (untested):
void mouseClicked(){
background = color(random(255), random(255), random(255));
}
- Besides, the usage of random at the beginning of your sketch is running only once. You need to repeat the random command as I just did.