I have brown hair which I dye as my roots are grey. I would like my hair to be a silver grey - is it possible to dye this colour? Or should I just let the roots grow out and spend 6 months of hell while I wait it out? I am 55 years old...

You'll most likely have to bleach your hair to get something close to a silver-grey color.

Remember that silver-grey hair often has some dark some white in it. You'll not be able to achieve the very finely divided differences in color.