Why Black Shell's back is green?

Did you notice black shell cordovan has green back surface?  More precisely, since cordovan is back side (or inside) of the horse hide, this green surface is actually the face side of the leather.

If you have unlined black shell LHS, it's easy to see this green surface.  It's sometimes vivid green and sometimes just faint green.  If there is no unlined LHS, another example might be the tongue of the shoes such as LWB like this.

Actually, I saw a lot of black shells waiting to be shipped at Horween.  Most of the black shells have vivid green back surface but some of them are just barely green.  I asked Nick why black shells have green back surface.

"After drying the black shells, we allow them to dry completely and then we rewet them in warm water so that we can remove excess dye.  After dipping many shells, the water becomes tinted by the dye that is coming off, which has the effect of dip-dying the backside green."

Horween is using "Anilin dye" which is vegetable-basis dye.  It seems their black dye is based on so-called "green-black" which creates black color from green as a result of some kind of chemical reactions, I guess...

By the way, if you see black shell under the strong light, you may see greenish shade inside black.  This is not defect nor aging result but the original dye content is just appearing...