It looks like the "weather product age" for NEXRAD weather data, i.e. how old the current data you're seeing is (see Pilot's Guide, p.13). The number is contextual, based on the weather information type:
The age of the displayed weather product—or the effective time of
Winds Aloft predictions—is shown in the upper right corner of the
display. For example, if NEXRAD is displayed, “0:05” indicates that
the data are five minutes old. If Winds Aloft predictions are being
displayed, “10:00” indicates the effective time for the displayed
prediction is 10:00 AM.
In the first image, the data is 15 minutes old, in the second it's 6 minutes old.
The information and images on p.72 show that NXRG is "Region NEXRAD" and NXCO is "CONUS NEXRAD". P.70 also mentions NXRD, which seems to be just "NEXRAD" and it's used when no data is available.
As for the color, the manual says:
When the age of the displayed XM Weather product reaches one half of
its expiration time, the color of the displayed time changes from
green to amber
NEXRAD data is good for 30 minutes per the Guide, and that matches with the color change at 15 minutes in the first image.
Note: although the manual talks about SiriusXM providing NEXRAD data, ADS-B radar data is also from NEXRAD. Presumably the display uses the same abbreviations, regardless of which service provides the NEXRAD data.