The Australian-made OEM panels I've come across over the years for the most part stick with orange for +24V, but are all over the place with -24V, sometimes also orange, or purple or grey or black or blue...
I would avoid blue (as it's also used as a live phase colour as well as, paradoxically, a neutral colour in flexible leads), and also avoid black for the reason mentioned above. Which reminds me of a call to a fault years ago - a new safety controller requiring a 240VAC supply had its "supply present" LED on but wasn't working properly. It had its neutral (black) connected by "bubba" in maintenance to the first black wire he found in the panel, which was a cable connected to a row of -24VDC DIN terminals. Which is to say 240VAC, limited only by the internal resistance of the controller, was being imposed on the 24VDC circuit that fed quite a few DC timers and devices. Somewhat surprisingly it didn't damage anything.