If you look at the metalizers, they have various shades, I usually go with the darkest but it really does not matter, at the end of your painting session they will look dark, not silvery. I use the sharpie on the white plastic and this gives me an idea how the panel lines will look at the end, that's just me being picky After I prime the car I will flow more metalizer paint into the lines, sand my primer and fix any imperfections and prime again. I stop flowing paint into the lines on my last primer coat, there will be enough paint in the lines for a good finishing effect.
Hope this helps..............