[Bionic]

Most of the things under the hood that are yellow are things you should put your hands on, such as the oil dipstick and all the filler caps. What are the yellow caps on the headlights for? The bulbs are changed by pulling off the rubber cap, so it's not for them.

 

[twospirits]

Your question peaked my interest so I asked Dave the Mechanic (on Youtube) and he says in the comments that it is for the adjustment for the headlight aiming.

TS

 

[bunburry]

Oh thats interesting. Do you have to loosen anything to get them to move or just use the yellow knobs?

 

[fmunk]

Oh thats interesting. Do you have to loosen anything to get them to move or just use the yellow knobs?

I'm no scientist or genius, but by the looks of it, it looks like a cover.

https://www.youtube.com/watch?v=BEdDYmC2gzo