At the risk of sounding a little controversial, I believe the younger generation of casters are rapidly moving towards a position where they don't massively care whether the headshots are in colour or black and white, so long as they remain accurate.
Colour is most definitely the sole default in the US; the UK remains more traditionalist, and there are, indeed, still casting directors who *prefer* to see black and white as a personal preference, but I find it hard to believe that anyone would refuse to accept a submission from you on the basis that you were proferring a clear and well taken colour headshot! For many younger casters, it is definitely colour that speaks to them, particularly for screen castings, as it it enables appraisals to be made even more quickly than would otherwise be the case.
Most photographers working on digital cameras will, these days, offer the option of giving you both colour and black and white versions of the shots chosen from the initial shooting session - though, as I understand it, this means they will default to colour and then alter the colour materials to black and white versions in post production. If you are printing out hard copies of black and white AND colour shots in separate print runs afterwards, you may be spending double on the prints; though using multiple versions uploaded directly to internet sites is generally unproblematic, and sidesteps the issue under discussion by giving casters the chance to choose which versions they prefer to view in a picture gallery.
Agency websites do *seem* to prefer utilising black and white imagery still, but even this is hardly set in stone (my agent seems to prefer colour since his website revamp). My advice would be to not, in fact, complicate the issue for yourself too much: if in doubt, ask a specific agency/casting director etc. for preference, but, by and large, I would say, any headshot that is accurate, shows you off well and honestly, is beneficial, regardless of the format it comes in. At the end of the day, casting directors still want to see a face they can cast, and gain a feeling of professionalism from the way it's presented, that's all.
