To highlight gray hair, make sure it is in good condition, then use a vegetable dye or semipermanent glaze to add the color. These dyes will stain the hair follicles.
Gray hair follicles do not always take color as easily as pigmented hair, so highlighting and dyes that stain the gray hair follicles look better longer while adding splashes of color among the gray hairs. The vegetable-based dye and semi-permanent glaze fade, but this reduces the need to redo the highlight's color as often. This is because the root line will not show as sharply between highlighting treatments and also applies to dyed gray hair.