Jhen Mohrans live in the Great Desert, where they swim in the sand. They feed off small animals that live in and on the sand. Doing this they also ingest large quantities of sand.
My theory is that all Jhen Mohrans secrete a glue like substance from their skin. This glue is purple in colour, but as Jhen moves sand and rocks stick to it, making him a sandy coloured outer shell. However, I believe that in times of food shortage, or from other enviromental hazards, a Jhen Mohran will hibernate under the sand for a long time, possibly many years. During this time the glue secreted from his skin pushes off the old, sandy coloured shell and replaces it with a purple one. This gives rise to the Purple Jhen Mohran. The reason for the purple colour could be that it is made from the mineral Carbalite, which in it's ore is purple. This means all Jhen Mohrans are the same species.
This shell is constantly replaced and so prevents the skin of the Jhen Mohran from becoming damaged by sand abrasion. When being replaced, the old shell peels off, which may be seen on the tusks.