Anyone who gets her hair colored knows that at some point the roots grow in darker, lighter or grayer than the color initially put in. A quick (& inexpensive) trick is to use Clairol's new Nice 'n Easy Root Touch-Up Kit ($7 at your local drugstore or buy it online here.
This permanent color & brush makes it easy to hide obvious regrowth (most prominent along your part). You'll find the product is thick -- a good thing because if this stuff drops on your bathmat, you'll find your hair will match it!
Don't want to purchase a special root touch-up kit? Try a basic all-over hair dye & apply it to your part with a clean mascara wand. Don't worry if your color doesn't match the hair dye exactly -- it probably won't. But don't worry, no one will probably notice.