Red among the yellow

Each vineyard parcel out back is made up of a single varietal, whether it's sauvignon,  cabernet, gamay, or any of the other varietals grown in our region. That makes sense. So you'd think that all the leaves in any given parcel would change to the same color in the fall. But there are obviously some vines in this parcel (and others) whose leaves are not the same color as their neighbors'.

Several reds among the yellows; a sign of less than healthy vines?

I have no answer for that. But I do have a theory (I am a guy after all and guys always have a theory). If you look closely between the red-leafed vine in the foreground and its yellow-leafed neighbor, you might see that a cane from the yellow-leafed vine appears to plunge underground and come up again as the red-leafed vine. That is a method of vine propagation in an established vineyard called marcottage (layering). There are a lot of vignes marcottées (layered vines) in the vineyards out back.

So, my theory is that marcottage may have something to do with the leaf color. The original grapes are grafted. That is, the French varietal is grafted onto a root stock (American in origin) that can resist infestation by phylloxera, an insect that ravaged France's vineyards late in the 19th century, and other maladies. But the layered vine is not grafted and when it grows roots they are its own. I wonder if the grafting, or lack of it, has something to do with the leaf color?

Naturally, I have no scientific evidence that this could be so. And my extensive quick and dirty research has turned up little on grape leaf color, other than that some normally yellow-leafed vines can turn red when they contract certain diseases. Could this be the case here? If anyone has the answer, I'd appreciate knowing.