Test molds, of which the factory made a few test pieces but never produced, are always desirable.
Test colors (marked as such with handwritten numbers or letters on the base or bottom of piece) are variable. The market for these is generally slim. In general, test colors on “popular” molds such as dogs, cats, or horses carry a higher value than others.
Factory errors are generally less valuable than tests. Pieces with errors such as unpainted eyes and missing gold on unicorn horns or hooves are considered “seconds” by the factory and are usually destroyed. Occasionally such errors escape the factory and are interesting curiosities, but they rarely command increased prices.