Natural Dark and Silver Hair

I think a purple shampoo might work. It is recommended for highlighted dark hair as well as silver, to lessen any brassiness.
My hair is dark with peppered silver rather than your attractive-sounding streaks, so I couldn't detect any difference when I used it.

I’m jealous, Sylken! My mother had very dark hair and a wonderful silver streak. It was when the Catherine Cookson series was on TV and we’d say she had the Mallen streak. Sadly I inherited my horrible hair from my dad....

Realising I wasn't going to go an elegant silver, I've been having ash coloured highlights for a couple of years now, very much better than the pepper and salt look. I use purple shampoo - so many on the market nowadays for all us oldies! See what it does for you sylken.