Just read the tutorial as it dose explain all of this
![Razz :p](./images/smilies/imported_icon_razz.gif)
In the case of an object like yours, which requires its own, unique texture, your best off just using the null_c.dds as your colour texture, and then having everything on the detail texture as this texture can only be used in one way, on one object.
It should also be noted that if your making a bunch of signs like this, all to be used on the same map, then they all should be on the same sheet, much like the roadsigns_PR.dds or the gazasigns_de.dds textures, to cut down on draw calls and memory usage.