Unity - Change opacity of objects around selected one

This is a change to the materials on those objects that should become translucent.

I'd start by creating "duplicate" materials of those in use, adjust the duplicates so they look translucent (which will require changing the rendering mode to transparent, then set the alpha channel of the material's color until it looks about right in an example.

If you have only a few materials you can prepare these in the editor and name them appropriately, so they are recognizable by name in code.

If you have lots of materials it may be better to initialize your code by reading through the materials of the model, create duplicate materials in code, and adjust them in code to be translucent versions based on the experiment I proposed above on one or two materials.

Then, at the moment you want objects to become translucent, you switch the material on that GameObject from the existing one to the translucent version (then back to the original when this state is ended).

To figure out how to do all of this, which is a bit more than a post should hold, start by reading the docs on MeshRenderer. There are example snippets regarding the material member. Note that if your meshes have multiple materials you'll need to use the materials member (plural) to get a list of all of them.

That last point is a per-object issue. Some objects may have only 1 material, while some might have multiple materials. Therefore, the script attached to the objects should perform this work, as the script attached to the object would adapt to the needs of each object.

Further, since this is a common behavior to multiple objects it would be best to consider creating a class that does this work and use it for all objects. How you implement this is up to you, but there are two basic approaches.

One is to "intercept" the classes Unity hands to you when you create or attach a script to an object. This means Unity hands you a class derived from MonoBehaviour. You will have created a class that is derived from MonoBehaviour with the code that manipulates and initializes translucent materials. You then edit the script Unity created for you, and change its base from MonoBehaviour to your "translucent material manager" class. The result is a class exactly like you had before, but the common behaviors that manage materials will be inherited by all classes you edit this way. Now you write code once for all objects that do this.

Another approach is to create a stand alone class that has the code for managing translucent materials, make a member of your GameObject that instantiates that for each GameObject, initialize this "translucent handler" so it "knows" the object it is attached to (the member will need some references to the meshes or other artwork that has materials to be changed), and have it do the work. Repeat for all objects. This is something like attaching a component to a GameObject.

To learn how to find material already in your project, look into Resources. You'll use Resources.Load to get a material from the resources, which presumably would be a translucent version, for use in switching materials on an object.

Also, you can study Material, a class you can use to create materials in code. You can read the material already on the GameObject (from Renderer, probably MeshRenderer if your model is a mesh), then use code to fashion translucent versions.

You'll also need to consider containers, like List, maybe Dictionary to help store these materials.

Your design might also create a stand alone "executive" class that coordinates all of this (something that is told what is the selected object, which ones should become translucent, and then how to undo all of this when it's over).