How change image color depth

[galievruslan]

I use Mini_magick with gem carrierwave. Trying to change the color depth, but it is not work. Changes only quality. How change image color depth using mini_magick

  def optimize
    manipulate! do |img|
      img.strip
      img.combine_options do |c|
        c.quality "95"
        c.depth "24"
      end
      img
    end
  end