It all depends on the natural color of your hair. If you are on the blond side of the spectrum, you can see a colorist about weaving in highlights that work with your grays and would require a periodic touch up, but would not be as visible as a contrast between dark hair color and gray at the roots. If you have dark hair and the grays are sporadic through your mane, then you can have a similar technique done as above, but the colorist would need to lift the color out of your hair and create highlights that would blend well with your grays. If you are more on the dark brown side, and your issue is the roots and you prefer a monochrome look (which is called a single process), then you can pick up a color of choice at the pharmacy to touch up your roots and then pull through your hair. I used to use Natural Instincts and that worked well for a single process at home dye job. It didn't seem to have the chemical smell other do. Personally, I think seeing a professional colorist is the best way to go to help manage your grays. Less risky, and they can work out a coloring plan that suits your lifestyle. If you do see a colorist, make sure you ask them about the maintenance of touching up the color job before you start. You will have to budget for it.
There are products to cover gray from temporary to permanent hair color, it actually depends on your natural color. If you hair is light then you would use a shade which closes matches your own. Of course you could always go darker. But if you are afraid to color your hair, as I am, then your hair stylist should be able to help you make the decision.
There is also a product called Touch of Gray, which gradually get rid of the gray and look more natural. If your gray is around the temples, as is mine, you can use 'Comb Away Gray' which is a color treated comb (in various shades) to cover the gray areas. I stopped covering my gray, it suppose to represent wisdom. Guess I must be pretty wise.