Nice find with the Gas Lamp, I am assuming you have the lighting column too. I cannot help with the numbering systems, as I recall they may have been numbered in situ too. Some were replaced as a matter of course due to damage or road works etc.
I do recall the corporation colours, but don’t have a definitive answer. Saying that, I do remember the newly painted lighting columns did vary slightly from the old. This may have been because paint manufactures did not always make the same colour range.
A possible scenario is the corporation may have used a BSI BS4800 accredited colour. This may have been known as the British Engineering Standards Association.
Here is a link to an article on the colour systems. The point of BS4800 is these were only 98 colours from the whole of the building indusrty, so your in with a chance of getting near right.
Hope you will post a photo and keep us updated on progress.
There is a museum of gas in Leicester, well worth a visit.