As a general rule there is one wire in the set called the identified conductor and it is required to be white - this is the neutral. Normally zero volts to ground.
When there are two different systems in an installation the wires will have two different colour codes. So in an RV, red and black are used to identify 12v and some other colour and white to identify the 120 volt. Green, or sometimes yellow, for ground. Ground wiring is strictly there for safety purposes and does not factor into powering loads.
In 3 phase systems (commercial/industrial) red, black, blue are the 'hot' wires in the 120/208v and orange, brown, yellow are for higher voltages (347/600 or 277/480). The higher voltages are used in building to enable the installation of smaller sized wires for the larger loads (or to be able to put more lights on one circuit etc).
Control wiring, on the other hand, doesn't have to have specific colours.
Probably more than you needed to know but maybe helpful to understand the reasoning on wire colours.
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