This example builds on the random circle exercise by constraining the random colour aspect of the sketch and only draws circles not ellipses.

size(600,600);
smooth();

int circleNum = 50;
noStroke();

for (int i = 0; i<= circleNum; i += 1) {
  fill(random(200, 255), random(100,200), random(100,200), random(100,255));
  float randomDiam = random(width);
  ellipse(random(width), random(height), randomDiam, randomDiam);
}

size(600,600);
smooth();

int circleNum = 50;
noStroke();

for (int i = 0; i <= circleNum; i += 1) {
fill(random(200, 255), random(100,200), random(100,200), random(100,255));
float randomDiam = random(width);
ellipse(random(width), random(height), randomDiam, randomDiam);
}