If you have DOT light (ie: US lights) the fog bulb has a yellow cover that snaps over the assembly to give the yellow beam. Most of the Euro lights have a while fog light. Cadmium coated bulbs will also emit a yellow light, but Cad bulbs are getting harder to find.
When driving, you really won't notice the yellow so much, not like you see when looking at the lights.
Best upgrade you could do is to convert to Euro lights if you are equipped with DOT lights. They make a MAJOR difference, so much infact that on the 560SEL, my low beams on the Euro lights are as bright as my DOT high beams were (from the driver's perspective) and I find I only have to go to high beams when well away from town.
If the driver's side tail light didn't brighten on the second notch you don't have rear fogs. That would be what the empty socket is for.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.