Every year, down at the corner from my house, there is a single morel. Every year and only one but it reminds me that the season is upon us. It's not necessarily the first but last year it got tagged by frost...but every year, one morel.

Every year it has been black. One single black morel. I've watched discussions about "blacks first, then half-free's then yellows." Or "It's been strange this season, the yellows came up first." All kinds of 'mythology and conjecture'.

The point I'd like to raise is that..
This year it was a Yellow. One morel, same spot but this year it was yellow.
So I'm going to float this possibility for your consideration and further discussion;

That the difference between Black Morels and Yellow Morels is the age of the mycellium that is producing the fruiting bodies...ie; the older that a particular colony gets, the more likely it will produce Yellow Morels instead of Blacks...

I'm hoping that you all didn't simply disappear now that the season is done...wade in with your thoughts.

They're different. What you're saying would be the same as assuming an old house cat might someday have puppies.

..or it could be like saying that for forty years MY hair on MY head has been coming up brown so it could never change to grey...but it has.
Is there a DNA difference between species? Is there any other way to explain the "Siamese Retriever" I saw this year?

DNA test results take weeks. I'll get back to you. As far as the hair on your head that you claim has been there for years, you are either a long haired hippy or you have plugs. In either case, the existing hair will not turn gray, only the new hair will start to lose color as it is produced. But you should use caution when swimming in pools. The Siamese Retriever you saw could be the result of it's mother just being one sick puppy.