Don't opt for a new box dye the very moment you notice gray strands instead of blonde hair. This shade might come off in a couple of washes, and you won't need to re-dye your locks. Do not use a toner or a toning shampoo if you think it might be a problem either.
If the graphite hue remains after several weeks, this is when you can return to your thought about purchasing a new box dye.
The best variants include golden shades, but if you decide to go darker, I recommend such hues as chocolate brown, praline, mahogany, and light brown to cover your gray hair.