Historically the difference was that yellow chromate coatings were heavier coatings than the clear ones, with more hexavalent chromium and consequently more corrosion resistance. Olive drab coatings were heavier still, for yet more corrosion resistance. The 'bright', 'blue', or 'clear' chromates were thinner and with less hex chrome either due to a different formulation or by virtue of being partially leached away in hot water after application.

A quick and intelligble document on this is "Quality Metal Finishing Guide -- Zinc and Cadmium Coatings" available from MFSA at www.mfsa.org for some nominal amount like $5-$10. It lists the salt spray hours to white rust of zinc as 12-24 hours for colorless coatings, 96 hours for iridescent yellow or bronze and 150 hours for olive drab. I infer that the hours to white rust for cadmium are apparently the same.

However, due to the worldwide pressure to get hexavalent chromate out of chromate conversion coatings, things are different already and will be more different tomorrow. For some time now we have had trivalent chromates on auto parts and they are inherently clear. In a decision I personally feel is ludicrous, platers and specifiers recently have been adding yellow dye to these trivalent chromates to try to make them look indistinguishable from the toxic coatings of yesterday. In short, until three or four years ago, color meant a lot but today it may mean a lot of hexavalent chromate or it may mean only dye on a trivalent chromated part.