I agree with Erich. Also, in some cases, it could be due to something as simple as available paint, pilot's preference, or a sense of 'display', where, for example, red might be used against a yellow background.
There were also various styles, with no firm evidence to indicate that these styles are 'official'. Some Abschuss zeichen might be painted as an outline bar only, or an outline with a date inside it. Others might be solid, with a date on it, or above it. Others may be capped by an arrow, or be in the colours of the country of the 'victim', or have the country's roundel above it, and so on. Some might just be plain white bars, if on a darker background, or, as seen on some night fighters.
There are examples of both black and red bars being used on the same 'core board' - the rudder of Adolf Galland's aircraft being one.