I often noticed that my Apple Watch flashlight would have a default red light setting instead of the lighter white light. I researched the web and nothing came up.
Fortunately, I found it serendipitous. It turns out that the white light is the default light, but if you turn on theater mode on the watch, the flashlight will automatically switch to the red one by default. This is to produce a softer light from the clock so as not to draw too much attention.
It's a small detail, but I thought it was really cool and well thought out.