we’d have all been wearing brown for the past or nearly 100 years
The reason why we wear/wore blue has got bugger all to to do with the colour of the sky.
HMG was in possession of a huge amount of blue serge which had been made for the Czar's Guard. It couldn't be delivered due to local difficulties at the time, despite having been paid for, so the stuff was available pretty much buckshee and was consequently imposed upon the upstart service.