The Breaker Finder 5000

I've been bitten by a neutral more than a few times.

I'm in the long process of installing 20A receps in my shop, arranged as one from each phase in a two gang box.
They will be spaced either one or two per 20 foot bay along the back wall.
I'm using white receps for the "black" phase and gray receps for the "red" phase.
I'm thinking about using yellow wire for the neutral of the red phase, with white tape.
If anyone knows the proper way- so I can differentiate at the recep.....
You can't "just" use the same neutral, or they're not truly "separate circuits".

...and I have a portable 240V compressor....and have considered making a 20A 240V female plug to two male 120V plugs, so I can "simply" plug that into one each of those separate phase circuits and have 240 anywhere along that 80 foot wall
There is only one neutral, you lost me on this. The neutral, however is not 180 on 3 phase like on single phase. So on 3 phase the neutral is tied to ground as always, but it is not 190 out of phase of any of the 3 hot legs

I don't know about codes, can't you just use the correct size neutral? (upsized?)

Or?? sounds like you are using individual TH wire in conduit? Can't you just string individual neutral conductors?