I like the resulting random color patterns. 

A small variation with 6 colors (the last transform part adds some black).

Images saved as a.png, b.png

 

size(400, 300);
loadPixels();
float rand = random(6);
int j = 0;
for (int i = 0; i < pixels.length; i++) {
  rand = random(6);
  j = int(rand);
  //print (j);
  if (j == 0)
  {
    pixels[i] = color (255, 0, 0);
  }
  if (j == 1)
  {
    pixels[i] = color (0, 255, 0);
  }
  if (j == 2)
  {
    pixels[i] = color (0, 0, 255);
  }
  if (j == 3)
  {
    pixels[i] = color (0, 255, 255);
  }
  if (j == 4)
  {
    pixels[i] = color (255, 0, 255);
  }
  if (j == 5)
  {
    pixels[i] = color (255, 255, 255);
  }
}
updatePixels();
save("a.png");
//
//transform1
println("transform1");
for (int i = 0; i < pixels.length-width; i++) {
  if (pixels[i]==pixels[i+width]) {
    pixels[i]=color(0);
    pixels[i+width]=color(0);
  }
}
for (int i = 0; i < pixels.length-height; i++) {
  if (pixels[i]==pixels[i+height]) {
    pixels[i]=color(0);
    pixels[i+height]=color(0);
  }
}
updatePixels();
save("b.png");