I recall reading in Spike Milligan's war memoirs, when his battery were in Italy, they received orders to repaint all their guns in the field — maybe not actually in the middle of a battle, but certainly on active service. He didn't go into details as to whether they were to be painted to a specific pattern though, and his colour description couldn't have been much more basic: he just said "yellow and green".
Those orders certainly didn't take any account of local conditions, because they had to use all the gunners' blankets to cover up their new "camouflage" colours and make them inconspicuous.
