My guess is that the answer to whether or not an inaudible frequency would have the “creativity effect” lies in whether the effect is caused by our consciously hearing it, or if instead it’s a subconscious effect. Just because our ears can’t detect a sound doesn’t mean that the sound waves don’t vibrate our cells, neurons, etc.

Who knows? Maybe when some artist answers the “Where do you get your ideas?” question with the “I don’t know – they just come to me out of the blue” response, it’s a matter of the presence of blue color/sound influencing the artist’s thought patterns at a level deeper than conscious thought.