It depends on your industry, brand and context!
Generally speaking, you’ll want to use a backdrop that supports your brand’s identity.
Working in financial services or law? Go for a dark or mid-tone gray backdrop.
Getting employee headshots for your tech startup? Go with a fun-colored backdrop or outdoors.
For geographically dispersed groups with locations in multiple cities, you should probably consider using a commonly available backdrop color. That way, you can use multiple local photographers to source identical headshots across teams.
Perfect consistency across all employee photos isn’t an absolute necessity. But, there’s something special about a website “About Us” page with perfectly consistent headshots across the whole executive team, executive board, or entire staff.