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 on menu button on the main screen of the app:
2. Tap on Settings:
3. Tap Brightness (HUD mode):
Two options are available, it's better to set it to Maximum: