How to slow down random color generator in Processing?

Hi everyone – I want to make a grid pattern of rectangles with random filling colors out of an array.I can get it done the way I want – but the random selection is way too speedy. I tried to slow everything down with frameRate(); – but this slows down the whole animation. (For example if I want to add something else). Then I tried to slow it down with if(frameCount%20 == 0) {…} but this does not keep the drawn grid – only lets it appear every XXX frames for one frame – does someone have an idea how I could slow down the lets call it "Color Noise"? – Thank you for any kind of help!

float size = 20;
color cbw = color(0, 0, 0);      //defines BLACK
color cg = color(0, 255, 0);     //defines GREEN
color cb = color(0, 0, 255);     //defines BLUE
color cw = color(255, 255, 255); //defines WHITE
color[] colors = {             //random selects one of above colors
cbw, cg, cb, cw
}; 

void setup() {
  size(1080, 1080);

}

void draw() {
  background(255);

  for (int x = 0; x < width/size; x++) {
    for (int y = 0; y < height/size; y++) {
      color c1 = (colors[int(random(0, 4))]);  //assigns a random color from above to c1-4
      fill(c1);
      noStroke();
      rect(size*x, size*y, size, size);
    }
  }
  }