how do you know: blue or silver muscovy

How can you tell whether a muscovy is blue or silver? We have a few pied muscovies, and one drake in particular seems to be a darker "grey" than hens and his lighter "grey" brother. How do I know for sure what we have? I searched for pictures online, but nothing seems to be definitive. Can anyone help me out?

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Thank you for your information. It was helpful.

I found some more pictures that I hadn't seen before on the links and otherwise. I saved some for future reference. I think that I have a pretty good idea now what we have.

While looking, I ran across this genetics table for muscovies. I have seen it before, but it wasn't of interest to me at the time. I was looking for pictures. Here it is:

http://www.muscovyduckcentral.com/genetics.html

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My blue muscovy have dark blue bodies with a darker almost black head. The silver is a very soft light grey-blue much like a sea gull color.