Walgreens isn't going to have much of a selection of charcoal grey nail polish....
Take the bike into an auto parts store that sells automotive touch up paint. You'll find lots of choices.
There are several brands - get one with a built in paint brush in the cap. Use multiple thin layers for best results. Let dry a few days between applications. After a month or so you can rub out the touch up with some rubbing compound or car polish.
Remember, your doing camouflage not a concours repaint. It just has to pass the 5 foot test - no one will noticed the touch up work from 5 or 10 feet unless they're OCD!
verktyg
Chas.