I had eyeshadow available to try out what shade of color would look best. Really use the products designed for this special purpose. I have heard about Joan River's products (QVC does great promotion for products). There are other products that are color fibers shaped like miniature feathers or pipe cleaners to clump and grab onto natural hair.
As for color... I liked the look of 1/2 to 1 shade darker on the scalp. Hair color does fade naturally, so a persons ends are naturally lighter. (or a perfect match) Problem is there are 9 shades of hair. Levels 1-3 are not discernable to the naked eye. Your color choices for these products are usually about 5 shades.
Permanent hair colors have different strength. As a rule the cream colorants (brushed on the hair) are used for full coverage; gel (from the bottle ~ but some hairdressers do use a brush) for more of a translucent coverage.
If you want to change your natural color, please stay within 2 shades of your natural level. The more contrast to your skin tone~ the older you will look. As a rule when that grey hair starts creaping in the skin has gotten lighter too.