Why black & white instead of colour?
1) As an abstraction from real life (i.e. colour removed), black & white can be more memorable, perhaps even stir more emotional connection
2) Removal of colour can have a unifying affect in a series of headshots (especially for clashing colours)
3) Focus more attention on the lighting and composition
4) Skin tones can, in some cases, appear more even and attractive
5) Instills an element of timelessness