You have a couple choices.
Paint it all silver.
Put a line of mask on it. (inner section center of scratch) then paint it gray, or a in between primer color (for example on boba its yellow).
Once that dries, put another coat of mask on, slightly larger, and paint your last colors (gold, brown, green etc). Peel the mask off. and it displays the silver, with a gray border, with the reg color on top.
The other option is paint it the colors you want, then do it topographical. Put the silver scratches on, then paint around the edges of it with a darker color.
This is a 'smoother finish'.
I prefer the mask method. If you do it with mask, you can dirty the helmet, and paint/dirt will stick around the edges of the paint and make it look more authentic.
Ciao.
Mason