I always found it very effective. We never bothered to find out if the colour code was for vis or cloudbase but just took the colour as read. If you had to have an alternate (diversion airfield) then it had to be above a certain colour. You only needed an alternate if your base went below a certain colour. Blue was lovely, white was good, yellows meant you'd be dodgy getting down some of the valleys in Wales, amber was a good day for instrument flying and red normally meant milk and two sugars.
Originally the idea was military instrument ratings were associated with the colour codes. If it was green weather then you needed a Green instrument rating to go flying. Somewhere along the line (before my time) that changed but the instrument rating colour system remained and, depending on whether you were a White, Green or Master Green determined the limits you could fly to.
As a Master Green (maybe it was Command Master Green) you could get airborne in 0/0 but you were supposed to know better. I do remember doing it once on the weather guesser's promise that things were improving ( and a necessity to test something out before we took the aircraft to sea the next day) but he lied (again). Unfortunately all the other airfields around my base decided to do the same and it was a very big relief when I shut down at RAF Brawdy with not very much fuel, having conducted a PAR to minimum minimums ( and slowing RIGHT down so I could get a better look for the approach lights)
AHHhhhh, this is great. 53 years old and boring like the very best of the clubroom old gits