a fireman just means a person has a license to operate High Pressure Boilers.

the license grades refer to the sizes of the boiler that the holder can operate..
some of the different classes of licenses are, in my state.

from lowest to highest

Boiler Tech/Second Class Fireman
First Class Fireman
Third Class Engineer
Second Class Engineer
First Class Engineer

Best shipmate I ever had 'graduated' from Red Fireman Stripes, to become a [Warrant] Machinist... a true "Snipe", "Daylight Dodger", member of "The Black Gang", "Bilge Rat", "Shaft Alley Lizard"... threatened with keel-hauling by the Leading Bosun's Mate if he EVER laid one greasy foot on that nice painted Main Deck...

COB

My grandpa said, "on a sub, everyone's a fireman." He was doin this kind of stuff when he was my age.

Yep. I should have specified light blue. They're pretty rare unless you hang around Port Hueneme. In six years active I never saw a set of light-blue stripes.

Thinking about it, service stripes and rating badges can also vary in color between gold and regular colors, depending on 12-year Good Conduct status.