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Hi Pierre
As this appears to have only one pair of post-sutural acrostichal bristles, I think it's more likely to be Neomyia cornicina (Muscidae). But the colour change applies to all these metallic green flies, including Lucilia (Calliphoridae) and Gymnocheta (Tachinidae).
As the fly gets older, the cuticle changes physically. I'm not certain exactly what the changes are - possibly the chitin layers of the cuticle begin to separate, but the effect is that the colour changes from metallic green to metallic blue or bronze or sometimes a metallic red. That this is an age-related change is evident from the condition of the wings in these old specimens (clearly shown in your top picture).
Other flies, such as Bombylius and Pollenia, lose their hair as they age, while in others such as Notiphila, the pubescence can change colour from silver to gold or yellow to brown.
It all makes identifying flies a real challenge sometimes! :D