Color every time. And don't even think about getting your headshots shot in black and white, even if you plan to use black and white later. Wait... Sorry if that was confusing. What I meant to say was:
Don't hire a headshot photographer who shoots the original images in black and white, even if you plan on using black and white images as your headshots. It's better to get someone who shoots color and then convert it to black and white later, this way you still have the option of going back.
Anyway, back to business. Why shot in color?
Headshots should be in color these days because there's no reason not to. Why would you want black and white? Casting directors need to easily see what your hair looks like. An example:
Is it red or dirty blonde?
Also they like to see your eye color. And if you have freckles. And many other things.
There is no reason not to shoot in black and white because these days no one has black and white Headshots anymore.
So in conclusion, opt for color Headshots!