Here are the results as of this morning at 10 AM (April 6). In photo #1 (four soaps shown above): The soaps on the left were colored with our bright neon blue liquid gel colorant and scented with Butt Naked fragrance oil. I selected Butt Naked because it has a 5.8% vanillin content. I tried to get a good mix on the percentage of vanillin in the fragrances tested. The soaps to the right (that look darker) were colored exactly the same as the soaps to the left. However, they were scented with non-darkening vanilla which has a 1.95% vanillin content. Oddly enough, it turned the soap much darker . There are no brown spots on any of the soaps to date but I want to let them all set for another couple of weeks to see what happens.
In the above pic (two clear soaps): Soap to the left that looks darker was our extra clear soap scented with vanilla (17% vanillin content). Vanilla stabilizer was added. As you can see, the soap does not have any brown spots (yet, and maybe it won't!), but, it has darkened. The soap on the right is our extra clear with no fragrance added.
(PS: There is this annoying little round black spot on the lens of my camera. I can't get rid of it. Think it's time to buy a new one. You will see it on the soap to the right. Sorry! I've tried cleaning the lens and that spot still appears.)
My final observation is that the vanilla fragrance with vanilla stabilizer added helped to prevent brown spots and browning, but it does darken the soap. You can see this in the above pic. The GPE white soap base has definitely turned slightly darker as did the GPE clear.
I'll give you another update in a few weeks. At this point, I'm not so happy with the non-darkening vanilla and will have to consider whether it's a fragrance I want to continue to carry. One of the other fragrances tested was our Peppermint. It has a 10% vanillin content. I was quite surprised to see that there was no color change in either the white or clear colored soap base. It seems that if coloring your soap, you may not notice any darkening of the soap unless you use the "non-darkening" vanilla fragrance. It's puzzling to me why that would darken and the other scents had little or no color change. Odd! Life is full of mysteries.