Should be easy enough.  One option: create an array of colours, set an int variable as the index and simply increment the index on mousePressed() (which registers clicks anywhere within the sketch window)...  though actually you may find mouseReleased() is a better option.  I'll leave it to you to find out why ;)

for a color range, its often useful to switch to HSB mode.

for example like this :


void setup() {
  size(600,400);
  smooth();
colorMode(HSB);

}

int col = 0;
void draw() {
  background(col,255,255);

}

void mousePressed(){
col+=20;
col%=255;
println(col);
}