Detect the first three primary colors from an image using the RMagick gem

colors.rb

require 'RMagick'
include Magick

def convert_rgb(color)
  color / 257
end

def convert_pixel(pixel)
  {
    :r => convert_rgb(pixel.red),
    :g => convert_rgb(pixel.green),
    :b => convert_rgb(pixel.blue)
  }
end

def is_different(pixel1, pixel2)
  margin = 40
  if (pixel1[:r]-pixel2[:r]).abs > 40
    true
  else
    false
  end
end

def get_colors(file_name)
  image = ImageList.new(file_name)
  histogram = image.color_histogram
  values = histogram.sort_by{ |k, v| v }.reverse

  first_color = convert_pixel(values[0][0])
  second_color = nil
  third_color = nil
  prev_color = first_color

  values.each do |value|
    color = convert_pixel(value[0])
    if(is_different(prev_color, color))
      third_color = color if !third_color && is_different(first_color, color) && second_color
      second_color = color unless second_color
      prev_color = color
    end
  end
  [first_color, second_color, third_color]
end

puts get_colors('cover.png')