There are numerous ways to get people's attention in an email, although, let's face it, people would rather watch YouTube videos than read emails. But if you work at the New Zealand firm ProCare, you may want to avoid anything other than basic black text. Two years ago, the company fired Vicki Walker for sending an email that highlighted a specific date and time in red, as well as an important message in all caps.
Walker won the suit, and rightfully so. The company didn't have a style manual, and on top of that, the email could hardly be considered confrontational. The sad thing here is that ProCare was probably looking for a reason to fire Walker, as multiple articles have suggested that she "contributed to disharmony in the workplace." This should prove once again that if you have a problem employee, it is critical to document the situation as time progresses, instead of looking for a quick solution.
Even still, there is rarely a reason to use all caps in a complete sentence. Maybe bolding a couple of words or headers is fine, but I would stick with black, unless you are using red to denote a blood drive or Jello wrestling.