The yellow bar indicates a Frame Error Rate error. These are also reported, with the actual packet error percentage, in the SITE SURVEY window found in the menu of the Belt Pack.
Frame error rate (FER) is displayed as amber or red if the percentage error passes certain thresholds, ie 10% and 20%
Since FER is a value from 0 to 16 the table looks like
Received Value (16 max)
It is indicated by a colour bar on the Wireless Status screen. Red is Error, Orange is Warning. No bar for normal