In order to get a brighter reflection when you're using the app in HUD mode, consider setting the phone screen brightness to maximum.
You can set it in the app, so that screen brightness is set to maximum brightness automatically once you launch HUD mode.
Here's how you do it.
1. Tap menu button on the main screen of the app:
2. Tap Settings:
3. Tap Brightness (HUD mode) and set it to Maximum: