Team color is defined by the model itself, not the skin only. It's something like an "event object (it's not, but I can't explain it better)". In order to remove the team color, open Warcraft III Viewer, go to View -> Find, get your Marine model and then go to Current File -> Extract MDX...
Open MDLX and convert the extracted model. It will convert it into mdl. Open it with Notepad (it does that automatically, most likely), scroll to Textures, replace the
Code:
Bitmap {
Image "",
ReplaceableId 1,
with something else or delete (be careful if you don't know what you're doing) [if you delete it, remember to set Textures count to Textures count - 1. If the model has 7 textures and you delete that, type 6 now.]
You most likely need to replace it though, because deleting it will also delete parts of the model.