Fixed! My resource id index number was wrong for my 2nd MATD.
Everything else was ok, but you only need one little thing wrong to break it!
Thanks to HP for her tutorial which helped me find the problem.

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I'm trying to make a recolor of a two part top mesh.
My mesh file works fine, but the recolor file gets borked when I try to add stuff for the 2nd mesh part. It doesn't even show up in BodyShop at all. Gory details below... Files attached... Haaaalp! Please!

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mesh parts
- top, yellow in picture
- top2, aqua in picture

Why two parts? The top was part of a full body outfit by fakepeeps7, used with permission. To make it work as a top, I either have to cram the UV map into a tiny area, squishing the texture into unrecognizability, or use 2 parts, 2 UV maps, 2 textures. Also, if I ever want to make an alpha kids top, this is good practice.

To make the recolor file, I recolored a working shirt (fakepeeps7 polo). Then I changed the CRES and SHAPE. The mesh shows up in BodyShop just fine. Of course, only the 1st part is recolored at this point. (see image below)

To color the 2nd part, I looked at a recolor of a Maxis dress with two parts, cfbodydressspring. Here's what I did with the polo shirt recolor from above:

changed GZPS
- number of overrides changed to 2
- added 3 lines for override1, part name is top2

added new TXTR resources for texture and bump map
- gave them new names where top was changed to top2
- fixed their TGI

added new TMXT
- changed name, top -> top2
- in Properties, made sure top -> top2
- in cMaterialDefinition, top -> top2, and SimSkin -> SimStandardMaterial
- fixed TGI (all file references look correct now)

changed 3IDR
- added a new line for the 2nd material, made sure that line had the correct instance and instance (high) of the new material
- and of course, already had the correct CRES and SHAPE for the mesh

When I added all this stuff to color the 2nd part of the mesh, the recolor broke. It doesn't even show up in BodyShop anymore.

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If you take a look at my files, don't use the 1part and 2part recolors at the same time, they conflict.