def to_xml_string(str = '', tag_name = 'color')
      serialized_tag('' + tag_name + '', str)
    end
This version does not support themes
def theme=(v) Axlsx::validate_unsigned_integer v; @theme = v end
Indexed colors are for backward compatability which I am choosing not to support
def indexed=(v) Axlsx::validate_unsigned_integer v; @indexed = v end
Serializes the object
@param [String] str
@return [String]