Custom validator for CSS hex color codes. I use the American spelling of colour as that's how it appears in CSS...

gistfile1.rb

class CssHexColorValidator < ActiveModel::EachValidator

  def validate_each(object, attribute, value)
    unless value =~ /^#([A-Fa-f0-9]{6}|[A-Fa-f0-9]{3})$/i
      object.errors[attribute] << (options[:message] || "must be a valid CSS hex color code")
    end
  end

end