They have an alpha channel. You'll need an editor that can edit alpha channels to edit it.
I just use DXTBmp and delete the alpha channel, as it's not that important, I'm not an expert, and I'm lazy.
To remove the alpha channel, just go to File -> Save as -> DDS Texture, then select DDS DXT1(No Alpha) option from the file type options.
Just remember to go to the editor and uncheck the Alpha Reflections option from that model.