Here at AsaLanta, there are many purple rocks.  I first discovered the very vibrant ones while digging up the mud pit for the adobe bricks.  I collected a good handful of them and decided to do some experimenting.  I crushed the rocks with a hammer, making them into a powder.   I then use the powder pigment as a plaster and a natural color pigment.  When I added water to the powder, it became very sticky… making me think that it is clay based.  So I added some sand to the mix and made it into a plaster.  I then tried it as just a color pigment mixed with sand and flour paste.  Both seemed to work well, however that vibrant color was not so vibrant anymore.  It mixes lighter and dries lighter.  The photo shows these options over a final plaster layer and a limewash coating.  I recommend walking around your site and seeing what natural pigments may be available.  Also the color of your clay can be just as beautiful.  At AsaLanta, the final coating is clay slip, flour paste and sand.  The clay slip being their pigment.  Get experimenting and see what you can come up with!