Sims wear default underwear when naked

I don't know if I can create a fancy tutorial.  But I can tell you the basic steps.  There are a lot of YouTube videos on sim 4 studio.  You just need the basics of how to recolor clothing to pull this off.  Nothing complicated.  You don't need to know anything about meshes or anything like that.

Install Sims 4 Studio.  Get the latest version.  It will be Star Beta something or another.

There is a studio and warehouse tab.  You need the studio tab and the CAS tab under that.  You can use the override option to create an override of an existing outfit or the standalone option to create your own version of it that doesn't override the original.  Then when you click on the CAS tab, you can search through the outfits in the game.

If you search for nude you can also find nude tops and bottoms for the sims.  If you hold down shift and left click on an outfit it will select all of the colors of that clothing item.  Under the studio tab, you can also look under tools and choose the option "Browse Game Content" and under that CAS.  That is for just looking at clothing in the game instead of creating a package file.  This has it's uses.  So, keep it in mind.

The basic idea of all of this is that you can pull up something from the game, make changes to it, and save your new creation as a package file.  You can then put that package file in your sims mod folder to add a custom clothing item to your game.

Now when you pull up a clothing item in sims 4 studio, there are different tabs you can click on to access the data associated with it.  Under the warehouse tab, you can scroll down and find Part Flags.  And under that a checkbox for ShowInUI.  Uncheck that box, and the item will be hidden from the CAS system, but it can still be available for use by the game.  Keep this in mind.  It might come in handy.

Now under the Studio Tab, you will find categories.  That will let you change things like what ages of sims the item is for etc.  But texture is what we want.  Under that you can look down and find a list of different kinds of maps.  Diffuse, Shadow, Specular, Normal, etc.  You can import or export these as PNG files.  And you have the option of creating a blank one.  Clothing items have a 3D mesh that defines it's basic shape.  You will find those under the mesh tab.  It item has like 4 of them to depict the object at different distances.  Then the maps we are talking about deal with the skin of the object and it's properties.

Because we are noobs, we are mostly going to want to play with these texture maps.  The diffuse map is the basic skin of the object.  And the other maps fine tune it a bit.  The shadow map adds shadows to it.  The specular map defines how shiny it is, what kind of shininess, and what parts are shiny.  The normal map shows bumps on the surface of the object for the purpose lighting it properly.  It makes it look more three dimensional.  These are all one PNG file each except for specular.  That's two PNG files.  One of them ends in .png.  The other ends in .mask.png.

What we are doing here is pulling up the nude tops and bottoms for our sims and saving the diffuse maps for them.  Then we pull up a default underwear item.  The salmon sexy underwear for a female sim, for instance.  Yeah, that's what the game calls it.  Then we import the diffuse map from nude into it and watch it disappear on the sim shown in sim 4 studio.  But if we don't want any weird shadows, we need to click on the buttons to make the other maps blank.  Shadow, specular, and normal.

Now we just save this override package and copy it to our mods folder.  And we have invisible underwear.  Just be careful not to mess with two many other settings.  Except maybe that one in the warehouse if like.  Otherwise, if you disable the underwear completely, instead of making it invisible, that part of your sim will disappear.  I've done this.  I know.