Yeah, I said "I thought" because I wasn't sure about that but was guessing. However, I did just go do some Googling for code sections, and I think I have found the section that specifies that. It's a section for designating ungrounded conductors in a branch circuit, which means hot wires.
It's 210.5(C)
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(1) Branch Circuits Supplied From More Than One Nominal Voltage System
Where the premises wiring system has branch circuits supplied from more than one nominal voltage system, each ungrounded conductor of a branch circuit shall be identified by phase or line and by system voltage class at all termination, connection, and splice points in compliance with 210.5(C)(1)(a) and (b). Different systems within the same premises that have the same system voltage class shall be permitted to use the same identification.
(a) Means of Identification. The means of identification shall be permitted to be by separate color coding, marking tape, tagging, or other approved means.
(b) Posting of Identification Means. The method utilized for conductors originating within each branch-circuit panelboard or similar branch-circuit distribution equipment shall be documented in a manner that is readily available or shall be permanently posted at each branch-circuit panelboard or similar branch-circuit distribution equipment. The label shall be of sufficient durability to withstand the environment involved and shall not be handwritten.
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Specifically look there in (1) (a). It lists four possible ways of identifying, and the first one says "separate color coding". There is a list I have seen somewhere before of allowed colors for ungrounded conductors, which includes black, red, orange, blue, and some others, but I think this place saying "separate color coding" means that they do need to be two different color wires for the different wires in the circuit.
But that's if you are relying on just the wire insulation color. The next methods listed are "marking tape" and "tagging". So I think you could go ahead and use black insulated wire for both, but then you would need to pick one and mark it with red tape or something at every end or junction point to mark which one is being used as NOT black.
So I think that's my best guess. You can't leave them looking exactly alike, so either the colors need to be different or need to mark one to show it's different. The (b) section seems to be about how if you needed to use some system of marking or identifying that wasn't the obvious black/red, then you need to leave a printed legend explaining what the markings are.