Licensed to Ill
As I touched on in my previous post, my personal "head canon" for Jorune is that the spell list is (for Shanthas) a training technique for young children. It's just that Muadra are sufficiently lacking in skill at weaving Isho that they're unable to progress past the training wheels of using a rigid defined list of specific effects, and so the Dysha "spell list" has become calcified as it passes from generation to generation in the Muadra community.Frankly I never thought the spell list approach made sense in the setting. If there is a version of BoL out there with psionics, you could probably raid that for a framework on which to build Isho. I'm sure there is something in PDQ that would work for Jaws.
Now on the flip side, if you want to be nice to Muadra, an extremist interpretation of the Shanthic organisation in to sects aligned to each moon would suggest that Shanthas tend to only have real ability at manipulating one colour of Isho, whereas Muadra work with all the colours (and sometimes even more than one at once).