The Ballad of Adler Young, Canto 2.5

I just remembered I have a question about a character:

Is Estvan Silverbrush supposed to be a red fox with gray coloured fur, or an actual gray fox (as in species)?

He's a grey fox; note that a few times in the Ballad, he's been shown to be an expert climber.

But he has children with Yolanda... And I'm almost 100% sure that Yolanda is a red fox.
And all the data I was able to find state that red and gray foxes can NOT interbreed.

So, just what did he do to manage the feat? Did he simply turn himself into a red fox during the "act"?
And couldn't Estmere use the same method (or have it used on him by Estvan, or someone else) to make an heir instead of having to rely on the Vulpitanians?