I suspect this is not possible, but I'm curious. In a matx shader, I can read a primvar and, for instance, run that into the seed of a randomcolor to get random colors on my instances. Can I do the same in COPs? Or is COPs strictly generating a 2d image and kicking that out? What I *want to do* actually is to bring in some id variable into COPs, send that into some noise seeds and 2D Transforms to get variation on a look, and reference that in a matx shader and have each instance get a different look, all from a single COPs network w/o writing out the variations. But my guess is there's no way?

You are correct, that sort of "live" material isn't possible with Material X.

The main workflow limitation is how bare bones the MtlX library is
Currently it heavily relies in COPs and textures to do even simple procedural stuff that were common to do in vex/vops

It's possible to do some simple noises and offsets in MtlX and one can still combine some COP patterns with whatever primvar based noises texture offsets and color variations in MtlX to achieve decent render time variations for instnances

But MtlX has long way to go to catch up what renderers normally support in shading graph in terms of patterns and workflows

My hope is for proper shading language for Karma to provide alternative for CPU only vex
It will of course not be replacement for COPs, but a lot of work can and has always been done in shaders without relying on making textures especially when the detail is screen space dependent or heavily relies on primvars which is common in FX

Got it. Thanks you two.