code: func randDeepColor() color.RGBA {
  
  	randColor := randColor()
  
  	increase := float64(30 + r.Intn(255))
  
  	red := math.Abs(math.Min(float64(randColor.R)-increase, 255))
  
  	green := math.Abs(math.Min(float64(randColor.G)-increase, 255))
  	blue := math.Abs(math.Min(float64(randColor.B)-increase, 255))
  
  	return color.RGBA{R: uint8(red), G: uint8(green), B: uint8(blue), A: uint8(255)}
  }
docs: //随机生成深色系.