There are regulations for DC voltages. Negative is BLUE and and Positive is GREY for all Voltages 24V and above. For Voltage UP to 24V you should use Black for negative and Red for Positive.
AC regulations require the same colours to be used as main where voltages are above 50V ac, below this all live conductors are recommended as normal (Brown, Grey or Black) with neutrals being Blue, but in control panels this can get complicated because of the number of circuits and sub feeds, so manufacturers tend to have specific cables they use all the time for power, depending on the type of circuit.
If this is a one off project i would recommend staying with Brown/Grey/Black for Live and Blue for Neutrals.
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