A yellow helmet of the kind that construction workers use, is the
hallmark of an entire class of enemies in the Mega Man
series of platform video games. Some walk on the ground, some
jump around, some fly, and others drive a bulldozer, but everyone
wears this characteristic yellow helmet. It is impenetrable to Mega
Man's bullets, and make the enemies invincible as long as they don't
take it off to look (which they often do with regular intervals).
The yellow helmet had become such an ingrained symbol of evil
throughout the series, that it was used as an in-joke in the intro
sequence of Mega Man VII:
Auto: Megaman, take this helmet with you. (hands
Mega Man a yellow helmet)
Mega Man puts on the yellow helmet. The Mega Man theme (the one
played after selecting a course) plays, then breaks off, in
"mid-sentence" as it were.
Mega Man: ........... (hands it back to
Auto)
Auto: I guess not...
Auto: Personally, I kinda like the other one...
(hands Mega Man his real, blue helmet, which covers most of his
head)
Mega Man puts on his helmet. The Mega Man theme now plays in full,
while the helmet is glinting.