I like to use white or yellow for "signals", but AFAIK there are no general standards.
I agree that there is no 'official' standard for DC circuits. At the refinery I worked at for decades they used 50 twisted pair black and white cables for most all their instrumentation junction box wiring runs, to and from the field/control house runs. Their standard? Black was positive and white was negative on all the 24vdc pairs. Why black for + ? because they wanted to follow the electricians standard for 120vac runs where black is the 'hot' wire and white the neutral wire. Go figure.