Insect pins have different coloured heads, with the heads being made of nylon (plastic).  Choice of a particular colour of pin can be based on individual preference, but also on the specimen being pinned, and the manner of displaying the specimen. 

Some people prefer the appearance of pins which have black heads.  Especially for specimens that are primarily dark/black as the pins are less obvious than the gold-headed pins under these conditions.

Black-headed pins are unique to Austerlitz.