Its under “hl2/materials/Debug” and it’s called “debugempty.vtf”
Get this: https://drive.google.com/open?id=0BwCFhuvzjH5sMjQ1T08tcjVEUm8
rename toolstrigger.vtf and toolstrigger.vmt to debugempty.vtf/vmt and replace them. It should just make the missing textures see-through.