I found this on the Bowser

I found this on the Bowser website.

http://www.bowser-trains.com/docs/instructions/C628.pdf

In the PDF it say,

"The circuit board (36) contains a DIPswitch that controls the color of the classification lights. Each unit is preset to display a green light while running forward, and a red light while running backwards. If you wish to display a yellow light instead of green, reverse the settings in the DIPswitch. The protective tape over the switch is not needed and can be discarded if desired"

Hope this help or what you are looking for.

thanks

Thanks that explains why the yellow class lights did not work.

Yellow Class Lights

Technically there aren't "yellow" class lights, only white and green.  They are probably using 3 color LED's so all they get is yellow (plus green and red).  The yellow is probably representative of white (extra train).