Are there any traditional colours to use for the wiring in a 4 mA - 20 mA current loop?
Traditional

Traditional standard colours for DC are red for positive and black for negative. In this case it could cause confusion and result in the black being grounded. To avoid this I would try red and white or red and clear, if available.

Current practice

Industrial sensor leads are now standardised as brown for positive, blue for negative and black for signal. That would leave you with a brown-black pair possibly leading to maximum confusion. If you supplied all three cores and leave the blue unconnected at the sensor end then that confusion would disappear but a new one would be introduced: where does the blue connect to?



Figure 1. Wiring for the ifm TAD981 sensor. Two-wire colour coding shown on left.

Selecting a random 4 - 20 mA sensor from the ifm range shows brown and white.