Bill,
Building on what Bjoern said, what you're looking at is not a Day/Night mode feature. You're seeing the default GPS 500 map background (black, with dark blue water) and it has nothing to do with daytime or nighttime. The user can display terrain elevation colors simply by clicking the TERR button.
The real Garmin GPS 500 did not have a TERR button. Its map background was black with dark blue water, just like the MSFT gps_500.xml default setup. Because of this, I'd encourage you to not make any changes to the gps_500.xml gauge.
However, if you want to set terrain elevation colors as your default, you, and your users, will need to edit the gps_500.xml gauge. Go to line 749 and change
Code:
<DetailLayerTerrain>(@g:setup_ColorTerrain) 0 == if{ 1 } els{ 2 }</DetailLayerTerrain>
to this:
<DetailLayerTerrain>(@g:setup_ColorTerrain) 0 == if{ 2 } els{ 1 }</DetailLayerTerrain>
That should do it.
Bob