I should also have said the U shaped raised area around the hasps was also painted blue.
It indicated that the Bombs 3"were only suitable for short range use. Each bomb only had 2 x 100 grain cartridges i.e. 12 per box. Later on the boxes were marked with the number per bomb instead of the total number in the box.
As I have already said, a piece of paper floated down from on high saying that certain changes were taking place from such and such. In reality when this was said, one Royal Ordnance Factory who had just finished a repaint job got a bid in to the paint suppliers and hogged the lot. Another ROF which was half way through a massive repaint job and had all the paint of the old colour to complete the job, found that when it finished the new paint was all gone, so carried on with the old paint until they could get a supply, but to keep working they got the other ROF to let them have any of the old colour they had left so that they could keep going. So some times things did not happen immediately.
I said before that in 1943 the colour of general boxes e.g. SAA, HE shell/mortars etc., which had no reason to be made distinctive for any reason, changed from the then deep bronze green colour to Service Brown No 2, but not overnight.
There are however some overall package colours which have to be distinctive because of the repercussions they could cause. If in WWII we had suddenly started to use Mustard Gas against the enemy, purely because someone accidentally issued some to the artillery by mistake, believing them to be HE, the repercussions would be enormous. Stocks were held in all ammunition depots in case the enemy started using it first, in contravention of the rules of war at that time.
To obviate this Chemical ammunition packages, then and now, are painted grey.
There were other significant package colours used. All smoke producing packages were painted Sea Green, as opposed to dark green, which indicates that your B166 package originally contained smoke bombs.
Dull Red indicated incendiary ammunition and Bright Red for Blank ammunition.