The Westfalia manual says that green is greater than 12.5v, yellow is less
than 12.5 v, and red is for less than 12 v.  This means that it takes very
little draw down to go to yellow, just disipation of the surface charge in a
short time after the engine is turned off may pull it down to yellow.  Based
on my experience with two Westfalias, '85 and '90, it's normal to see the
battery indicator drop to the yellow area and still have a lot of power
left.  When you get into the red and it stays there you may have marginal
starting juice left.  I've left those lights tied to the starter battery and
use a second added on voltmeter for the aux battery.