On the VexLabs site, they have posted a 2006 FVC detailed field drawings document (
http://www.vexlabs.com/docs/detailed_field_drawings.pdf), as well as other documents of interest. On the final page (63), in a field assembly diagram labeled "Autonomous Field Lines", it says to "Tape Lines as Shown using 3/4" Wide WHITE Vinyl Electrical Tape".
VexLabs (IFI) is a sponsor of this season's FVC competition, and is supplying official parts for field construction, so I expect that this information is accurate (and certainly much more detailed than has been seen previously).