A method of staining a poly(amino acid) by contacting a poly(amino acid) with an overall neutral or overall cationic complex of at least one of transition metal ion, and a plurality of donor ligands each of which is fully coordinated to the transition metal ion and is either a nitrogen donor ligand or a cyclometalated donor ligand, such that at least one of the donor ligands is a cyclometalated donor ligand. Nitrogen donor ligands will contain heteroaryl ring systems having from 10 to 40 ring atoms, wherein each nitrogen donor ligand is substituted with from 0 to 4 R1 groups. Cyclometalated donor ligands will likewise contain heteroaryl ring systems having from 10 to 40 ring atoms, such that at least one ring atom is N, wherein each cyclometalated donor ligand is substituted with from 0 to 4 R1 groups. R1, R2, R3 and R4 groups are defined herein.