I don't think he is remarking wire colours. He is only identifying them at both ends.
Most of the time number markers or wire lablels are used. They wrap around the wrie at both ends so you can identify which is which.

As an example if you had five black wires you could label them 1-5 at both ends.

As long as you are simply marking them for identification (not NEC color code) you can do just about anything you want.

There are rules that govern the 're-identification' of wire colors, but most are not allowed in residential wiring with a very few exceptions.

Switch loops should have the white re-marked to indicate it is not a neutral.
You can also re-mark a white as a hot in a 240 V circuit.

Marking a black as a white is not allowed, nor is marking a white as a ground generally acceptable in smaller conductors.

Yes, I'm basically asking about using just black for all hot-side conductors, white for all neutrals. Sure it's great to have red and blue for add'l branches in the same conduit, yellow for switch loops, etc., and the whole happy rainbow, but in the DIY quantities I'm looking at, it's a diff between 6.2 cents/ft. and 34.1 cents/ft... no need to work up a spreadsheet on that price delta, lol.

The question about using white as green was just a curiosity, not an issue in my app, which is metal boxes and EMT conduit. The only ground conductors I need are 8" bare pigtails, and that's an easy amount of insulation to strip from solid Cu wire.

Thanks much for the clarifications.

Just strip any color handy and use a bare ground.

The only time you would have an issue is if you have an isolated ground in a metal box, and that is not a common thing in residential.

If you are puling into conduit you can use colored tape to mark different circuits.

Just avoid orange. It is used to mark high legs in three phase.

No bare grounds in pool/spa wiring either (which you WILL find in residential).

"Can wire w/ white insulation be used as GND if green tape at each end?"

NO.

NEC requires equipment grounding conductors size 6awg or smaller to be:

"identified by a continuous green color or a continuous green color with one or more yellow stripes on the insulation or covering, except where bare."

Just avoid orange. It is used to mark high legs in three phase.

Check. That must be why my Gardner-Bender multi-color asst. PVC elec. tape blister pack includes just about every color BUT orange. ;')

NEC requires equipment grounding conductors size 6awg or smaller to be:

"identified by a continuous green color or a continuous green color with one or more yellow stripes on the insulation or covering, except where bare."

Double-check. Thanks for the citation. See above.

REALLY don't enjoy re-wiring, and wouldn't do it just to upgrade older TW (60C) wire with THWN (90C) conductors, but bloody hell, when a "hot" is a 12AWG blue wire at the panel (15A breaker), and emerges at the first j-box as a 14AWG red, well, that's redneck code-violating intra-conduit splice I will NOT tolerate. No doubt trying to conserve scrap wire, lol. Similar low-IQ "craftiness" abounds.

#4 and larger can be reidentified pretty much at will with tape. Green tape makes it ground, white tape makes it neutral. The reason being, most wire in that size is just black and wire that size is expensive, so getting lots of different colors becomes costly (the main reason most of it is just black).

There are few actual NEC requirements for multiple colors for multiple hots/phases. Customarily, we use black, red, and blue for different hot phases or 3/4-way wiring for 240VAC or less. Higher 3-phase voltage uses Brown, Orange, and Yellow with Grey neutral,