Collect array to hash

eddie.rb

#!/usr/bin/env ruby

input = [["GREY", "1"], ["GREY", "2"], ["GREY", "3"], ["LIME", "1"], ["LIME", "2"], ["LIME", "3"], ["PINK", "1"], ["PINK", "2"], ["PINK", "3"], ["RED", "1"], ["RED", "2"], ["RED", "3"]]

out = {}

input.each do |a|
  out[a.first] ||= []
  out[a.first] << a.last
end

puts out