How do you change the materials of linked instances stored in another blend file? I'm creating roughly 15 product models with several color variation as well as product sets.
I am having a problem as one product can be used to sell in a set with other products for example, or one product can have up to 8 color variations.  As we've continued our work, I'm asked to make a tweak to the model itself, or perhaps the color is a little off and I need to tweak the color, now I have a nightmare of a problem because I have to go and locate every single product model, and update several models, and several materials across multiple blend files!!
So I'm learning about creating instances and linking to the product models and materials stored in other files.  So for example, I now have each product model, stored in it's own blend file.  I also have a separate blend file which stores all the materials.  I'll go to File&gt;Link, locate the product blend file and link the collection from that file that make up the model.
I do the same for the materials, File&gt;Link, locate the materials blend file and select all the Materials in the Materials folder.
Now I have another problem.  I have no way to change the materials of the linked models!  In fact!  There is no material panel at all!
I do also see the materials available if I create a sphere for example, and I can change the material to one of the linked materials.  So I know they have imported and still remain linked.
I can also go into the master blend files and make changes to the model and materials and they update!
So I'm half way there.  I've figured out how to import the linked models and materials.  But I cannot figure out how to change the linked model's material.
So to clarify, I need to know, how do I &quot;unlink&quot; the material from the linked model only, and change it to one of the link materials I've imported instead?  All while being able to make tweaks to the model and material master blend files and have them update properly?
This is what worked for me.
I have two master blend files.  One containing the materials, which are specific to the product.  And another one of the product model.
When I create a new blend file to create a scene with the product using the materials, I need to be able to update the master files and see them update in the new blend file. For example, when I show the final scene to the client and they want you to make a tweak to one of the materials, or a tweak to the product model, I would be able to go to the master blend files, make the tweaks and any other blend file that is linked will automatically update.

This is how I set it up:
First I need to create a link to the model or material

Then I navigate to the master blend file for that model and import it.  Here I have the product saved as a collection, so I select the collection the model contains:

I repeat the same steps to import the materials, except this time I navigate to the materials folder inside the blend file and select all the materials I want to link:

Next I make sure the model is selected and while in Object Model I go to Object&gt;Relations&gt;Make Library Override and confirm:

Now I can access the materials panel, but in order to update the material to one of the linked materials I have to switch it from Data to Object:

Now if I click the icon to change the material, I now have access to my linked materials and can update the models material in the scene:

Now if I need to make a tweak to the material or model, I can open either of those master blend files, make the tweaks and save.  If I'm working with the current scene and master files and have two instances of Blender open, I can force an update by doing the following:
First I change the outliner from View Layer to Blend File:

I scroll the outliner to the bottom until I see the two linked files, indicated by the link chain icon.  I right click one of those and select reload.  Then it updates the scene to reveal the latest tweaks!