``Copper-washed" 9mm. cartridges

[P.P.C.eer]

Can anyone tell me the idea/purpose behind ``copper-washed" 9mm. cases?
I got some in a batch of surplus ammo many years ago..and have always wondered their peculiar colour, and the reason behind it.
They are Headstamped I V I 75 .

 

[green]

They are proof cartridges, an extra high pressure round for checking the strength of a 9mm. DO NOT fire them!