I think that you're being a bit too hard on Mr. Gamblin...there really is no particular standard as to what "Naples Yellow" should look like.
I gather the pigment for "Naples Yellow" was originally found in the earth around Naples, Italy. I also imagine that there are naturally occuring variations found in each pile of dirt dug up to get the stuff.
"Naples Yellow" is a term as general as "Apple Pie"...there are as many recipies as there are cooks to make it (and my mom's was the best ever...)
Fortunately, we each get to choose our favorite.
And I don't particularly like Gamblin's Naples Yellow either...