Cross-posting from my Forrst: http://zurb.com/forrst/posts/Collecting_colours_from_an_image_Ruby-Lgo

colors_from_image.rb

    require 'rmagick'
    number_of_colors = 100
    file_path = "/path/to/file.jpg"
    begin
      temp_file = Magick::Image.read(file_path).first.quantize(number_of_colors)
      pixels = {}
      pixel_count = 0
      temp_file.each_pixel do |p,c,r|
        pixel_count += 1
        pix = p.to_color(Magick::AllCompliance, false, 8)
        pixels[pix] = (pixels[pix] || 0) + 1
      end
      puts "Pixels: #{pixel_count}"
      pixels.sort { |a,b| b[1] <=> a[1] }
    rescue Magick::ImageMagickError
      {}
    end