In a universe where sound waves are visible as colorful trails, Zephyra stands on a hilltop watching her friend Kofi, who is some distance away across a valley, play a trumpet. The colors of the sound waves show the pitch of the notes: high pitch notes are red, and low pitch notes are blue. Zephyra is colorblind to red and can only see blue. Kofi plans to communicate a message to Zephyra using high (red) and low (blue) notes on the trumpet. The message is a simple one: if Kofi plays a high note, it means danger; if he plays a low note, it means all is safe. How can Zephyra accurately understand Kofi's message, given her colorblindness and the way sound waves are perceived in their universe?
Respond briefly.
Given that Zephyra is colorblind to red, she will not be able to see the high pitch notes that are red. This directly implies that if Kofi plays a high pitch note, it will be invisible to Zephyra. However, she can see blue, so low pitch notes will be visible to her. Therefore, when Kofi wants to communicate with Zephyra, he could play the trumpet using the following strategy:

1. If there is no danger, and all is safe, Kofi should play a low pitch (blue) note which Zephyra can see.
2. If there is danger, Kofi should play a high pitch (red) note which Zephyra cannot see.

Since Zephyra will see a blue trail whenever Kofi plays a low pitch note, she will know that it means all is safe. Conversely, when Zephyra cannot see any color trail (because the high pitch red note is invisible to her), she will know that it signifies danger. Thus, the absence of a visible sound wave will act as the signal for danger, and the presence of a blue sound wave will communicate safety to Zephyra.