Whenever dusting a fingerprint with the fingerprint powder, it must be kept in mind that the color of the powder must contrast the color of the surface. When a fingerprint is discovered on the dark surface, then the fingerprint powder used must be of gray color, so that the print is easily visible when developed. The light color of gray will help the ridges of the print to stand out over the dark surface.
Hence, the correct answer is 'gray.'
