Welcome Enrico glad to have you join us and thank you for sharing.
You will find very few things in painting have absolutes as to approach, there a many ways to reach the same results.
But, to your questions:
- Priming there are various schools of thought though white as an undercoat is pretty common with the idea that using thin overcoats allows the colors to shine through being again common. It is also common to undercoat in enamel or similar if you paint in oils. But, I have read of undercoating in light grey and black for the purpose of enhancing shadows so who knows. (Greg DiFranco and Peter Fernk also undercoat in a grey or brown to get a feeling for the shadows and the colors are laid out which is fascinating - we have articles from both if you search the site.)
One approach used successfully by a number of folks is to undercoat in the correct colors using one painting medium - enamels and then oils - Mike Taylor and Nicholas Ball or undercoating in acrylics and then overpainting in thin layers of oils.
(You also have some amazing folks that use exclusively acrylics for amazing results - Greg DiFranco, Penny Meyer, Peter Ferk - it is really a matter of what you are comfortable with.)
Gianpolo BIstulfi's site has a nice painting tutorial as well as a LOT of amazing work, you might try here as well:
http://www.zinnfiguren-bistulfi.com/generalita-dic14.htm- Shadows - again different approaches. One interesting article is Daniel Canet - also on the site - titled Painting in Vivid Colors which has some bearing in this area.
- Metals - Matter of choice, some folks polish the metal of the figure itself, variations in grays are very common and everything in between
- Pictures - I have the same issues as you though I have been told by very 'good photographers' that they just use an IPhone and do nothing. I suspect paying better attention to the lighting and or getting light box probably would help in this realm.
There is a knowledgeable of articles on How To advise as well as some great stuff in the BFFS Journal but, both of those areas of the site will require becoming a member. However, the group here is always willing to share/assist and encourage so always feel free to enquire.
The figures you showed were quite nice and your interest i expirmentation will pay dividends. It is really a matter of finding what works for you.
mark