To tell you the truth, I'm not aware of a specific colour code for 24V DC wiring.
I've seen a number of different systems in use, from red & black (much the same as 24V DC vehicle systems), to brown and grey (with either polarity depending on where you get it from). Each has its advantages and disadvantages, depending on the installation and what other equipment is installed in the vicinity.
Red & black isn't really ideal without lots of labelling as it can also be used for 240V AC. Same goes for the three phase colours.
Is it a specific order where you can ask the customer what their standard is?