at first i had doubts about it, but then i realised that i.e. hemocyanin which contains copper is achromic when in its reduced form and blue when oxygenated. chlorokruorin is green (and contains chlorine - the same effect as in chloroplasts), aso. therefore it would be logical if it were iron what gives hemoglobin its red color - the heme is obviously the thing which causes it, and searching through this structure only iron seems to be proper answer.

of course i might be wrong

must correct myself:
when i wrote about oxygenated form of hemocyanin i meant its oxidised form. i still don't really well know chemistry vocabulary.