Is there any industry-defined guidelines to select the color of LEDs which is used for power indication in systems? A colleague told me that red color LED is usually used for 12V, green for 5V and orange/amber for 3.3V. I couldn't find any relevant results in the web while googling for that also.
For industrial equipment, IEC 60073 might apply, which reserves red indicators for dangerous conditions or emergency situations. For example on machines with stack lights.
IEC 60073 Basic and safety principles for man-machine interface, marking and identification – Coding principles for indicators and actuators
There are only the common IEC, ANSI, EN, etc. standards for industrial applications. They usually don't apply to anything outside that field, so if your application doesn't have to comply lets say EN60204-1:98-11 (or any other standard), then you can use whatever colours you want but check any requirements first.
But just for the fun: IF your application would need to comply with EN60204-1, all the LEDs you mentioned would have to be green: (Red = Emergency; dangerous condition / Yellow = Abnormal; check physical dimension for exceeding limits / Green = Normal; physical dimension inside normal range / Blue = Forcing; action required / White = Neutral)
And after all, the standard rule in electronics (as mentioned before) summarized: