by chemical method

[profmania]

i really wanna color diamond by adding pigment not by irradiation
such as mercuric oxide i hear it color diamond

 

[JohnQuixote]

Do you want to enhance it short-term or long-term? Diamonds have great stability, so numerous approaches to influencing apparent color are explored.

You can use a felt tip pen to color the girdle and just below and even that will change the color. There are many kinds of coatings, but anything on the outside of the diamond is temporary. HPHT and irradiation are more permanent, depending on the subject and the treatment.