Hi Paul,

I believe that where you collected the shells is the key to the unusual
color.  Not too far from where I live is the Cape Cod Canal, the widest
manmade canal in the world.  Both sides of the canal are lined with
large rocks, and the dogwinkle, Nucella lapillus, is common there.  All
the dogwinkles in the canal are white, except at two locations - on the
concrete supports of the two large suspension bridges that pass over the
canal.  Here they have the same reddish brown color you described.  The
source of the color is iron, which washes down from the bridges, and
gets incorporated along with calcium into the shells.  You said the ship
you were diving on was mainly wood construction, but I'll bet the iron
framework you mentioned is the source of your shell coloration.  You
described it as "rust-colored", and you were right on the mark!

Paul M.