Yellow is commonly used as one of the 277/480 colors, just like the grey often is for the 277 neutral.We use 18 AWG Stranded TFFN, pulled in with power conductors in conduit:
Its rated 600V, and listed under 725.49(B)
I would prefer a label 'Class 1 system' rather than the sharpie blackout.
I would encourage you to pick obvious colors that are not used in your building.
There was a tenancy to use grey and purple which I have seen mistakenly connected to neutrals.
I spec yellow and purple.