Why r y b phase in sequence?

Answer for USA, Canada and countries running a 60 Hz power supply service.If you are referring to line phase or branch circuit conductors which have colored insulation or are marked with color tape, then R (red), Y (yellow) and B (blue) do not normally occur in the same panel.

The undisputed industry standard for the above is as follows:

Three phase Low Voltage panels (110/208-240volt) are marked as follows from left to right as you look at the panel or other electrical control/distribution:

Phase A conductors are Black, Phase B conductors and Red, and Phase C are Blue.

Three phase High Voltage panels (277/480volt) are marked from left to right as:

Phase A conductors are Brown, Phase B are Orangeand Phase C are Yellow.

Single phase line and branch conductors are not color coded.

Note that the National Electric Code does not specify conductor colors with the exception of the grounded conductor, (the neutral) which is to be have insulation or marking tape which is to be white or grey. The industry standard here is that white is used for low voltage neutrals and grey for high voltage. The code also requires that the earth ground, or grounding conductor is to be green or bare copper. The only other specified conductor colors in the code are for conductors on the secondary side of an isolation transformer panel in an hospital operating room. Chances are you don't need that information.

Answer for countries in Europe and other world areas running a 50 Hz power supply service.In the UK red, yellow and blue were the standard colours used for labelling the line wires in a 3-phase circuit until 2006 when the Wiring Regulations were changed so that the standard colours are now brown black and grey to conform with the standard colours common in mainland Europe. These colours, however, are stillstandard in the electrical supply industry. The sequence of these colours also defines the standard phase sequence of the supply.

For more information see the answer to the Related Question shown below.