Local Switching
Black - common
Gray - switching live
Brown - Live
I take it this is with reference to a 2-way (=U.S. 3-way) switching arrangement using the "extension" method? (i.e. supply to L1 of switch, lamp to L2, and a 3-core linking L1-L1, L2-L2, and Com-Com on the other switch).
I can't say I've noticed an emerging preference for this yet, but then I don't think there was ever really an established convention as such under the old red/yellow/blue colors either.
When wiring that method I would use red for supply (L1), since that seemed logical given that on a regular switch drop red would be the feed. Then I would usually use blue for lamp (L2), and yellow to link the commons.
Transposing red/yellow/blue to brown/black/gray respectively does result in brown for supply (L1), gray for lamp (L2) and black for switch commons as per your list, but I can't say that I've seen that established as a real convention yet, anymore than my use of yellow/blue was the only method employed under the old system. If I found 2-ways wired "extension" method with blue for the common and yellow for the lamp, then I'd wire mine the same way for consistency.
If wiring a 2-way the traditional method, either to junction boxes above or by splicing the feed or lamp return in the back of a switch, then I would normally use red for the feed and the lamp return, with yellow and blue for the travelers (a.k.a. "strappers" in the U.K.). Transposing that to the new colors results in brown for feed and lamp return with black and gray for travelers.
If using a 3-core for individual 2-gang switching, then I think it's much more obvious. Red (now brown) for feed, yellow (now black) and blue (now gray) for the two switch returns.
Of course, under BS7671 the yellow and blue conductors were supposed to be sleeved red at terminations anyway, and now black and gray should be sleeved brown at terminations when used in single-phase wiring.
