I'm not aware of any herbal dyes specifically to turn hair silver. If you like doing herbal muds (like henna or cassia) and you want something that won't deposit color over your silvers, you can try sedr!

As far as enhancing your silvers, the only thing that I think would work that won't harm your hair would be something like Manic Panic. It's not an herbal dye, but it's a deposit-only vegetable-based dye that is very gentle on the hair- in fact it's quite conditioning! I helped my mom transition from dyed hair to her natural silver/white and we used Silver Stiletto as a regular hair mask to enhance the color and to help re-condition her hair while it grew out. They also have Alien Gray and Blue Steel, which are both lovely silver-gray colors.