For grotesque idols the texture preview shown during building doesn't match the outcome, in regards to the secondary wood texture.
Aside:
But lets do a real example:
Primary:

That is certainly nice, I like blue after all.
Secondary:

Heck yeah, more blue is always good.
Now, using a sketchpad on a preexisting idol's secondary wood texture (primary wood was cypress in this case) and a piece of beaver salvage, we see:

Which isn't the same as the build menu's preview. So minimum, there is an inconsistency here between the two preview methods. But lets follow through and build it now, to be thorough.

So yeah, that's all I got. Build menu preview shows wood interior texture when the game (and sketchbook) actually applies the bark texture.
Tested this on both vanilla and amber clients.