For mains wiring there are mandatory colors; for your own low-voltage wiring on a project, there are no requirements. I usually (but not always) use the following scheme:
Positive supply voltage (e.g., +5V, +3.3V, +12V, etc.): Red
Ground: Black
Negative supply voltage: Blue
Digital logic signals: White
Analog signals: Yellow
Special signals I want to quickly locate: some other color like purple, orange, grey, etc.