Actually, they have done runs of walnut shell. So, they aren't limited to the three colors you listed.
However, lighter shell is difficult to find lately, and that's being discussed in the Alden thread. Lighter shell demands better quality material, because flaws are more evident.
Now, if the process for producing bourbon shell resembled the process for calf to any extent, bourbon wouldn't be any easier to obtain than walnut. That's because bourbon is burnished walnut.
So, while it would be nice to get some lighter shell, I wouldn't hold my breath.
AE is out of Walnut shell as of now and they don't plan to restock it in the future. You are right that they have done runs of walnut shells, but they ran out of it in August, or so they say.