nicely done I needed some more simple and common clothing for my little boys (and those maxis are... well...)

Making untucked tops with two mesh groups certainly is nice for recolorists for the reason you mentioned, you needn't squash the UV map and can use existing Maxis textures. However, it creates some extra work for the recolorist as well, the removal of duplicate textures / bump maps so as not to waste disk space. Since your recolors have duplicates, I'm guessing you might have forgotten to do this step before uploading? Please either tell recolorists how to do this, or direct them to the link below, or another similar tutorial.

how to remove duplicate textures

I noticed that while your tees do have a base, it is colored completely transparent in your textures. At least, it's transparent in the grey tee, I didn't check all of them. What do you see in game when you look at the base? Sim skin? Or whatever color the texture has there with the alpha channel ignored? The base is in the "alpha7" group, but this is not really an alpha group. It does not have different opacity than the top group, it is SimSkin, not SimStandardMaterial, and it does not have alpha blending enabled. That's just fine if you didn't want it to have transparency, but then I'm puzzled as to why it is named alpha7 and colored transparent.

If you want them to see the inside of the tee, I would either turn backface culling off and remove the base completely (which I wouldn't recommend), or add an "inside" layer to the mesh, with normals pointing inside. It needn't be the entire tee, that would be a waste of polys, it could just be one row of triangles, a cone with its point in the sims chest (your base with the point pulled up). Regardless, the base or inner layer would need to be textured.
sidenote: this tee is not the only untucked tee mesh with a base, it differs from some other meshes in having a base, but not all

I know some Maxis bottoms clothing, and several custom bottoms I've seen, map the shoes/feet close to the area where you mapped the tee base. I'm not certain if your base overlaps the silly Maxis shoe/feet area, it might not, but it would be worth checking. I prefer to map untucked top bases to the same area as the "outer layer" of the tee shirt, so they get the exact same texture, "inside" and "outside". Or you could move the base mapping to the top of the image, to avoid any potential UV map conflicts with bottoms. It wouldn't affect your current recolors since they don't have the base textured anyway.

Looking forward to more untucked separates from you in the future.