After many fruitless searches I now have a definate answer to the blue or green lens on Chalwyn and Kenyon lamps provided by a senior engineer in the electricty industry . Whilst these are of the same design as the Red and Yellow danger lamps they were made for the Central Electricty Generating board in the Uk. The lamps were from electricity substations, they were originally painted red even though they had green lenses. So when people were working on equipment , some parts of the sub would be switched out and other parts live, at the time we are talking about 1940’s to 50’s maybe in to the 60’s. The voltage we are talking about was 33.000 and 66.000. The dead part, that is the part to be worked on would have metal pins and rope around it and the lamps with the green lenses would be placed inside this area. Lamps with red lenses would be hung on various structures on the live equipment, red for danger. Now this was ok at night for although possibly not much work was carried out then, if an engineer had to make an emergency visit he would know what was what but during the day the green lamps would not be visible especially in bright sunlight so some bright spark (forgive the pun) thought, if we paint the lamps with the green lenses green then not only would they be recognisable in the safe area it would be easier to pick the correct lamps for the safe area. They were superseded by cones and danger notices, cones with a green flag stuck in the top for safe zones and tie on and wrap around danger notices on the live structures, making sure of course that you didn’t tie them too high. The blue glass used by Kenyon with the yellow flame gave a better green colour, much teh same as the blue lens used in railway signal lamps
Cheers
Steve