Color is added to select strands of grey hair using a variety of methods, including pulling hair through a cap and coloring it, painting color directly onto the hair, and via a process called color weaving. Colorists apply lighter colors rather than dark ones if a highlighted look is desired.
It is chemically impossible to highlight grey hair. Highlighting is the process of removing pigment from hair, but grey hair doesn't contain pigment. The general public often uses the term "highlighting" incorrectly. Colorists blend away grey hair through lowlighting, which is the process of adding pigment to select strands of hair.