With the first gray hairs, a good option is a demi-permanent color, such as Schwarzkopf's Igora Viviance, that covers gray 70 percent. Because they do not lift the natural color, visible roots are insignificant as the hair grows. The color is durable enough that it performs like a permanent color; not a temporary one. You will experience minimum fading between salon visits. Another option is Essensity, which will provide most with 100% gray coverage. Like Viviance, it is ammonia free, but covers up to 100% of gray and unlike a demi-permanent, it offers the option of going lighter.
With more gray hair, a natural lightening effect is taking place. Many of us have seen persons who've decided to color their graying hair in order to look younger only to have the opposite results. For most of us, as we age, our hair color (and skin tone) typically gets naturally lighter (regardless of gray) and the color we had naturally in our twenties may be too dark when we're fifty. By coloring the hair with a too-dark hair color we only succeed in making ourselves look pale and emphasizing any age lines and wrinkles we may have. When your goal is to cover gray hair, it is often wise to aim for a color a bit lighter than the natural color of your non-gray hair. Additionally, with a lighter color you can avoid at least some of the harshness of white roots. 
Igora Royal or Essensity are both great permanent colors for even coverage when there is more than a little gray hair. When the hair is resistant, Schwarzkopf Igora Royal Absolutes are made for this purpose( shown in the image above). This product provides even color results on more than 50% white hair with 100% color intensity.
Highlights & Lowlights
Highlighting will hide a certian amount of gray hair. Lowlights can also reduce the amount of gray hair by roughly 25%-30%. One solution that works well is adding an all-over color between the highlights. (See the next section on highlights). A demi-permanent color between the foils of the highlight can make a big difference in covering gray compared to just a highlight, as covering the gray 100% does not matter as much because of the variation in tone. A demi-permanent works well here because it does not create as obvious a line of demarcation as the roots grow out.
Color Placement
Gray or salt and pepper hair can make you look older than you feel. With reduced pigmentation, nature may also be lightening up your hair in variable degrees of uniformity. Fortunately, variations in the distribution of white hair can be beautiful and expressive, and you can may choose to enhance this effect without returning to the exact shade of your younger self. Adrian is highly skilled at creative color placement on gray hair, paying special attention to hair around the face and the parting to minimize the appearance of roots. One advantage to keeping the light areas lighter is that roots will be much less noticeable, keeping you looking better longer between salon visits. See "Accenting with Placement" under the creative tab for more information.