What primers for my multi-primer Contrast/Speedpaint/Xpress charts? (Patreon)

Hey there, so one of my projects for autumn and winter is to make swatches for Contrast, Speedpaint and Xpress Color over multiple primer colours.

I thought of: White (White Scar spray), bone (Wraithbone), grey (Grey Seer), slapchop (black to white drybrush), and gold (Retributor Armour). 

I also want to have a silver, and now I'm asking you, what kind of silver would you prefer?

Plus: If you have any other suggestions, please drop them in comments!