These were provided by the printers before the final colour was chosen. They are probably uncommon rather than rare. Being visually attractive, they are therefore much sought after, making a useful addition to any collection. The $2 is available in at least a dozen different colours or shades. Similarly, the $5 and $10 were seen in many colours and some are yet to be listed. A comprehensive description is given in part 1 of The stamps and postal history of North Borneo by L H Shipman.
Some of the colour trials are also available without perforations in the centre. There is at least 1 centrally perforated $5 with separate cancellations in both halves which led onto a suggestion that there might have been some other use for these adhesives, for instance, as telegram stamps.
It is amazing how fresh and new they look after more than 120 years!