drawCirclesを参考に。

def setup
  size 600, 600
end

def draw
  colorMode(HSB)
  dia = 60
  len = width/dia +1
  background(255)
  smooth()
  grid len, len, dia, dia do |x,y|
    fill(random(360),150,200,200)
    strokeWeight(random(20))
    stroke(random(360),150,200,200)
    f = random(15)
    ellipse(x, y, dia-f, dia-f)
  end
end