class Module

Person.class_variable_get("@@hair_colors") # => [:brown, :black, :blonde, :red]

end
include HairColors
class Person

end
end
[:brown, :black, :blonde, :red]
mattr_writer :hair_colors do
module HairColors

Also, you can pass a block to set up the attribute with a default value.

Person.new.hair_colors = [:blonde, :red] # => NoMethodError

end
include HairColors
class Person

end
mattr_writer :hair_colors, instance_writer: false
module HairColors

instance_writer: false or instance_accessor: false.
If you want to opt out the instance writer method, pass

HairColors.class_variable_get("@@hair_colors") # => [:blonde, :red]
Person.new.hair_colors = [:blonde, :red]
Person.class_variable_get("@@hair_colors") # => [:brown, :black]
HairColors.hair_colors = [:brown, :black]

end
include HairColors
class Person

end
mattr_writer :hair_colors
module HairColors

allow assignment to the attribute.
Defines a class attribute and creates a class and instance writer methods to

def mattr_writer(*syms)
  options = syms.extract_options!
  syms.each do |sym|
    raise NameError.new("invalid attribute name: #{sym}") unless sym =~ /\A[_A-Za-z]\w*\z/
    class_eval(<<-EOS, __FILE__, __LINE__ + 1)
      @@#{sym} = nil unless defined? @@#{sym}
      def self.#{sym}=(obj)
        @@#{sym} = obj
      end
    EOS
    unless options[:instance_writer] == false || options[:instance_accessor] == false
      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
        def #{sym}=(obj)
          @@#{sym} = obj
        end
      EOS
    end
    send("#{sym}=", yield) if block_given?
  end
end