example of how to convert image to "-format %c histogram:info:" (http://www.imagemagick.org/Usage/files/#histogram) by gem "mini_magick". It used below to get dominant color of image by this solution for ImageMagick http://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=12033

mini_magick#get_dominant_color.rb

require "mini_magick"
 
module MiniMagick
  class Image
    def get_dominant_color
      color = run_command("convert", path, "-format", "%c\n",  "-colors", 1, "-depth", 8, "histogram:info:").split(' ');
      # color = "   1764000: (208,195,161) #D0C3A1 srgb(208,195,161)\n\n"
      {
        hex: color[2], 
        rgb: color[1][1..-2].split(',')
      }
    end
  end
end


i = MiniMagick::Image.open("/home/a.jpg")
 
p i.get_dominant_color()[:hex]
p i.get_dominant_color()[:rgb]

Thanks, it is really useful,
I just updated my usage to get Hex code using Regex,

I think Regex will be much accurate,
hex: color[/#[A-Fa-f0-9]{6}|#[A-Fa-f0-9]{3}/]