When using MC, AC -- which are bundled factory colors -- the Code permits #6 thru #14 to be re-identified. Think switch-legs.
That option does NOT exist for EMT runs where it is obvious that the correct color can be pulled in at no extra expense.
Of late, there is a trend to use fully covered feeders. Very handy.
On all commercial jobs out my way we see mixed 277 and 120 volt grounded distribution. Yes, separate colors and MC are ordered.
Because the neutral for the 120 run is 30 degrees delayed behind the 277 neutral -- they CANNOT run on the same conductor. That's why the Code has the OP's noted provision.
BTW, delta and wye diagrams come from Tensor Mathematics. When you drop the tensor diagram for a Wye transformer inside the Delta tensor you can see that the three legs split the interior vee's in half. So...
Each Vee = 60 degrees
Cut in two = 30 degrees
So when delta waves are transformed into wye functions they lag by 30 degrees -- because the media is an inductor.
If, for any reason, these neutrals get crossed over you get terrific harmonics on them as 'circulating current.'
