I use all kind of colorants to soap, some natural some synthetic. Sometimes I run into situations where color morphs into an undesirable hue, or fades during the curing process. It's very common that natural colorants fade in time because most of them are light sensitive. I can make a batch of soap with the most vibrant beautiful colors just to find them unsellable at the end. Let's see the runner ups.
Here is the soap made with Spirulina (a type of algae). Spirulina is a deep green powder that smells fishy and when it's added into soap and then forced gel it produces this vibrant intense leafy green color. Then a few weeks later that vibrant leafy green turned into yellowish olive green.
Here's another one using natural colorant, Gromwell root infused 72% olive oil soap. Gromwell root is widely used in Asia mainly for herbal medicine. It is infused in oil and butters to make slave or balms to treat various skin conditions. It can also be used as fabric dye for purple color. Soap color made with Gromwell root infusion can range from blue purple to plum purple depending on the harvest. Again, the beautiful intense color didn't last...
Now this final one surprised me. I've used this specific colorant before and never have it fade on me in such a short period of time! It turned to barely anything in just 2 days! Yellow #5 lake is a strong yellow dye, it's synthetic, not natural. I suspect the fragrance I used for this batch has something to do with the yellow disappearing.
What colorants have you tried that don't last?