Oh that was a great series of articles by, I think, Michael Bowyer. I believe "duck egg blue" was the eye-witness term used at the time before official paint specs were formalised, and referred to the squadron-level repainting of in-service fighters during 1940 as fighting intensified. How I wish I still had those magazines today.
It was indeed Michael Bowyer. The articles were eventually revised and expanded into the books "Fighting Colours" and Bombing Colours", and were an invaluable reference in my plastic modelling days; I've still got "Fighting Colours", but sadly in storage & inaccessible at the moment.