A small bash script for outputting the most used 5 colors in an image.

palette

#!/bin/bash

for c in $(convert $1 -colors 5 -depth 8 -format %c histogram:info: | sort -d -r | cut -d '#' -f 2 | cut -d ' ' -f 1); do
  colors+=($c)
  convert -size 50x50 'canvas:#'$c /tmp/$c.png
done

color_files=
for c in ${colors[@]}; do
  color_files="$color_files /tmp/$c.png"
done

convert \( $color_files +append \) `basename $1.output.png`

palette.rb

#!/usr/bin/env ruby

rows = 2
colors = 10

file = ARGV[0]
command = %Q{convert "#{file}" -colors #{colors} -depth 8 -format %c histogram:info: | sort -d -r | cut -d '#' -f 2 | cut -d ' ' -f 1}
hist = `#{command}`.split("\n")

def generate_line(filenames)
  '\( ' + filenames.join(" ") + ' +append \) '
end

color_files = hist.map do |color|
  color_file = "/tmp/#{color}.png"
  `convert -size 50x50 'canvas:#'#{color} #{color_file}`
  color_file
end

output_file = "#{File.basename(file)}.palette.png"
lines = []
per_row = (colors / rows)
rows.times do |x|
  min = per_row * x
  max = per_row * (x+1)
  lines << generate_line(color_files[min...max])
end
`convert #{lines.join(" ")} -append #{output_file}`

And a slightly updated on in ruby. This is ugly!