An openness toward sharing with the camera should be the primary objective for the subject. When actors give me what I need to work with in this area, I can produce a considerable volume of great shots in a short time. Strong eye contact plays a large role.
I 'm a huge fan of both black and white and color but BW is currently out of fashion for use as acting headshots. Back in the days of film black-and-white was cheaper and easier for making adjustments, so that was the industry standard. Since digital has made it affordable to retouch color, the industry has switched over completely. I always shoot color - they can be converted to bw when preferred.
For wardrobe I generally recommend form fitting, darker, solid colors but to maintain a contrast with your skin tone. I ask my clients to avoid anything with a lot of extraneous ornamentation such as ruffles and articles with asymmetry .Textures generally register nicely and light layers such as spring or fall jackets can work very well. Logos are generally distracting and undesirable and unless you're a character actor with perhaps a hipster or hip hop character look. Leading men and women should avoid logos. Otherwise, your wardrobe style should reflect how you'll look at an audition.
In terms of style, make-up for leading types should usually be a natural "look" and should reflect how you will appear at an audition. Character types usually want to explore a more distinctly individualized style and accessorization.
The most important technical facet of makeup for actors headshots is to ensure that skin is not shiny. Make up artists use a powder or foundation and possibly an airbrush to reduce. Shine is typically more of an issue for indoor studio sessions with strobe lighting than it is for outdoor and/or natural light sessions. While you want to look natural, you also want to look your best. Blemishes will detract from the impact of your headshot - it's important that the focus of the viewer is on your expression and not on any part of your skin or wardrobe. The amount of retouching required will vary greatly from shot to shot depending on their subjective preference, the lighting, their type of skin and amount of blemishes.
Digital retouching offers an extremely powerful option and can effectively help to make most any corrections that you may want to make. Even when your objective is a natural appearance, it can look unprofessional to have an image with stray hairs and blemishes. These types of issues only serve as negative distractions... Retouching is not a new development. Back in the day, retouchers would airbrush paint directly onto the photograph and then that photograph would be photographed and then a batch of prints would be made from that. Flaws in BW prints could be mitigated with dodging, burning and contrast.
The size of a face in an 8 x 10 actor head shot is printed at a ratio that's fairly close to actual size so you perceive every detail that you would see in real life, and probably then some. Since the objective in an actor head shot is a natural appearance, there is small segment of people with excellent skin and meticulous styling that can get away without any or with only minimal retouching. However, almost all digital images can benefit from some amount of post-production. Even if retouching is not necessary, fine adjustments to color and contrast can greatly strengthen the impact of a headshot.
Small grooming details are perceived as large in 8 x 10 sized prints. Hair should be well organized, intentional, clear of the eyes and usually should be in the style that you'd will wear to auditions. The basic rule for women is to wear hair down and natural. Men should be very well groomed. Many grooming issues can be retouched at a cost - but it can laborious and time-consuming. Hair spray or gel if necessary is recommended to minimize messiness.
For leading men and women, jewelry is usually best kept to minimal or none. Some women, particularly with short hair for instance may feel they look naked or boyish without earrings or some other jewelry - perhaps go with a small stud earring or a very thin chain. There are common exceptions to the "natural" look - for instance, charcter actor types shooting for roles outside the mainstream will often suggest their "type" via their headshot styling with jewelry makeup and/or accesories.
Backstage also offers a comprehensive volume of information on Actor headshots advice.