Rails carrierwave mini_magick - how to change color depth?

Question

Ruby on Rails

Hello
I am using carrierwave + mini_magick to upload and edit images. It is necessary to change the color depth for uploaded images.

def optimize
    manipulate! do |img|
      img.strip
      img.combine_options do |c|
        c.quality "95"
        c.depth "48"
      end
      img
    end
  end

Only the image quality changes, but the color depth remains the same. Tell me how to be?