FBX Uploads but shades my whole character one color?

1 Fbx doesn't shade your model, its the colour setting placed on it in Blender that is doing that i.e, you set a default colour and that is what Mixamo is reading

2 Mixamo is a rigging software so its just adding bones so the colour you see in Blender still stays the same

3 don't send people your .blend file... its worthless for sharing [we need Fbx or Obj + textures]

4 your model needs a neck