The two existing specimens were originally preserved in alcohol, but though this can discolour specimens, it is not probable that it could turn green to blue. Hume and the Dutch orhinthologist Hein van Grouw have also suggested that due to an inheritable mutation, some Newton's parakeets may have lacked psittacin, a pigment that together with eumelanin produces green colouration in parrot feathers. Complete lack of psittacine produces blue colouration, whereas reduced psittacine can produce a colour between green and blue called parblue, which corresponds to the colour of the two preserved Newton's parakeet specimens. The reason why only parblue specimens are known today may be due to collecting bias, as unusually coloured specimens are more likely to be collected than those of normal colour.