down here we used the red as common, sometimes come across the yellow used as it but not muchas above. use grey as N is standard practice to disassociate the black from neutral, thanks to the meddling of johnny foreigner. as for 2 way switching, back in the days of sensible cable colours, most of us used the yellow for common and R/B for strappers so that in the switch with the switch wire from the light, you has red with red and blue with black. this equates to black for yellow, brown for red and grey for black