Columbia subject headings:
Purple--Congresses.
Symbolism of colors--Congresses.
Harvard subject headings:
Color (Philosophy)--Congresses.
Dyes and dyeing--Congresses.
History, Ancient--Congresses
Purple--Congresses.
Purple--History--Congresses.
Purpura (Mollusks)--Congresses.
Symbolism of colors--Congresses.
Princeton subject headings: