1. Download magos model editor from the tools section.
2. Open your model. Check on it a bit - does it have parts where it seems like gaps in the mesh has been made by the texture rather than by modelling? In that case, you are limited to changing only the parts where the model already has teamcolor.
Otherwise, if the model isn't using any alpha channel, you can just open the texture editor, right click>"create empty". If you right click>edit the texture that appeared, you will see a list of explanations on what number represents which replacable ID; in this case, you want it ti be number 2, since that's eamcolor.
3. open the material manager, and open the material that seems to have the unit's main texture. There, set the filter mode to blend.
4. Create a new material layer in your selected material, and set it's texture path to the path of the teamcolor you just added. Click ok, and then move the new layer up.
If you did things correctly, you should now be able to add teamcolor by editing the textures alpha channel.