The answer is ‘it depends’…
Fear of the dark is a known fear and so indicates Mimulus. But part of the fear could be fear of ‘something unnamed’ in the dark, or of something that you cannot name happening to you while you cannot see – and that is an Aspen fear.
In the same way, if you hear a noise and think that it might be an intruder, then that would be a known fear and you would take Mimulus, or Rock Rose if you were truly terrified. But if the fear is purely imaginary – you check the house and find nobody, but still feel afraid that ‘something’ is there – then that begins to be Aspen.
In practice if you can’t decide between the two it may be right to take both at once, since elements of both fear can be present at the same time.