car.rb

module Lib
  def self.my_attr_reader(klass, attr)
    eval %{
      class #{klass.to_s}
        def #{attr.to_s}
          'red'
        end
      end
    }
  end
end

class Car
  Lib::my_attr_reader(self, 'color')
end

c = Car.new
c.color