Pigment Powder for Basing?

hey Everyone i got a Question hope fully someone here has an awnser too. im Using AK interactive Dark Earth for my Bases for my Space Marines. i dont know what Pigmente powder i should use to Put some "dust" on them i never used Pigments the only one i own is a metalic That i got gifted. also i need to seal the powder in right? do i just use a varnis? if yes what typ of Varnish my Space marines are Metalic