My way to do sept markings (I use bone for the base and white for the highlight)
I think theyre similar to what you're after
I do two very thin lines of bones with a fine detail brush
Then I do a line next to each line, slightly higher than the one next to it
Then I slightly paint over the taller line with white, I think it might work for you, Also your fire warriors are beautiful so im sure you're more skillful than me and could make it work far better with two shades of orange.
Ill upload pics of two of mine in a sec, not the best just taken with phone and a lamp
Basically to sum it up, keep it simple, keep it clean. Less is more with tau
imo. It looks like you've not done enough but suddenly it pops and looks great.
Realised I missed the sept markings on dude number 2's rifle... :l