Blender model missing colour when uploading to Mixamo

1 Mixamo shows gray because it knows you have a texture but can't read the file... by design Mixamo can read jpg or Png images but will show gray if you load for example a tif

2 just fyi Mixamo displays white when it can't detect any image at all

the small model will be because you didn't make a standard Fbx i.e, when Mixamo converted it some of the data was lost... another common mistake is models that have more than one file type i.e, the face is Fbx but the body is Obj

how to fix always = make standard file types [harder with Fbx because autodesk keeps changing their mind when deciding what their standard is]

if you want to share the model (with textures) then i'm happy to try for you