different oils add color as well. Linseed being more orange, Tung or Soy more yellow. Poppy can be quite clear. And tree resins vary widely in color too. Add to that several decades of sunlight on the wood and you get a kind of depth that can't be replicated out of a can. I wonder if any of the makers might have used a shellac sealer coat under the varnish, I suspect that when I see alligatoring on an old finish.
As an aside, none of us knows what these boats looked like when new. A lot of people were horrified when they saw the results of cleaning Renaissance paintings. The muted and warm colors became vivid and nearly garish -- exactly how they looked when first painted, but our idea of them was created by looking at centuries old colors covered by layers of soot, dirt, and darkening varnish.
A big problem with oil painters is the darkening of the Linseed oil, hence the desire for Poppy oil.