Hi Joseph,
That Tricolor has a true yellow center! And I think there is another Ipomea that is yellow. If I were breeding morning glories, I would be aggressively cross pollinating in order to get yellows (and related oranges, creams, and such) into the Morning Glory world.
You have mentioned morning glory crosses that weren't possible in the past, and I have no doubt that people have made such reports. But I wouldn't accept them as gospel. There is that old saying that,
if at first you don't succeed, try, try, again.
There are new techniques for making crosses that were previously not successful. I wouldn't be breeding zinnias if they didn't include a spectrum of warm colors. Cross pollination, and making hybrids between hybrids, can produce new genetic combinations. I have seen many such recombinations in my zinnias, and these are just a couple of examples.
I'm not saying that you personally should take on the task of of creating yellow morning glories, but I think that some morning glory breeders should do that. Morning glories are capable of a lot more than what they have shown for the last few hundred years. And so are zinnias, and probably a lot of other ornamentals as well.
ZM
