Hair color product for gray touch-ups?

Recently noticed my first gray hairs and am looking for recommendations for hair coloring products. I've always liked my natural hair color and so have never used any products, not even for highlights. My husband uses Just For Men but that's a one-time-use product, he has to use all of it since after it's initially mixed it becomes useless pretty quickly. I only have a few gray hairs so far (knock on wood) so I'm not looking to dye all of my hair, just want to target those few little troublemakers, to do touch-ups on a rolling basis.
Can anyone recommend a good product to use and reuse? All advice greatly appreciated!