[FrankJ]

The standards are some what conflicting.

IEC likes Blue for zero volts. The NEC likes blue for non zero DC volts. A good many panels in the US will use Blue for +DC & Gray or Blue with white stripe for 0V. The AC parts of the NEC call for white for neutral or grounded conductors & Black for non grounded AC conductors (Highly simplified). In DC Black for zero volts & Red for positive is pretty universal. Except Black & White twisted pair is really common for shielded control wires. Some plants like using black for the 0 V which makes white positive. Others use white for O V (It is the "neutral" color) which makes black positive (hot). The DC 0 V often gets grounded somewhere in the plant, making the black wire the grounded conductor. (A big NO if you are on the AC side of building wiring.) The take away here is unless you are somewhere that is really rigid about following one standard: you need pay attention to where the wire is going in addition to the color of the wire.

I would like to see the rule for wiring colors relaxed for multi conductor cable going to sensors & other low power items. The 2012 rule still requires the sensor supply wires conform to the color code. I can see still requiring the color code to be followed on single conductor wire.