Hi,
the reason why I noticed the tagging: colour:pole and colour:support are not in use, but pole:colour and support:colour are.

But there is also a logical reason why it makes sense. The main feature is a sign. It has several colors, so subkeys are used. colour:text, colour:back as you mentioned. 

The support and pole are additional features which are not strictly part of the sign itself, so all tags concerning them are grouped in a namespace with all tags starting with pole:* or support:*. 

We do the same in many other cases, e.g. highways:   The highway gets a "surface" tag, and the sidewalk gets a "sidewalk:surface" tag.