Paths with colons work fine for me, but I am still using Sierra, not High Sierra.
If this doesn't work, it's entirely possible that this is due to some change in High Sierra. The fact that it continues to work in Panorama 6 means nothing. Panorama 6 continues to use the old Carbon API's, which only support the colon format and know nothing about the UNIX format with slashes.
Panorama X has special support for the colon format. If it detects colons in the path, it internally converts them to the / format, which is the only format used by Cocoa. In other words, only Panorama understands paths with colons, other Cocoa programs do not. This internal conversion isn't just a straight replace, there is special handling for the volume name. This code in Panorama X is several years old, and hasn't been changed since May 2013 -- over 4 years ago. In other words, this code definitely did not change in any recent Panorama X release.
It's possible that High Sierra has changed something in one of the APIs getting called by this internal conversion routine. Perhaps this has something to do with the conversion to APFS -- is your drive a SSD that converted to APFS? I reviewed Apple's 10.13 developer notes and don't see any documented change that should affect this, but it could simply be an Apple bug -- after all, we are talking about an entirely new filesystem here.
Does the info("volumes") function work for you? This uses some of the same code used for converting from the colon to / format.
Do you have more than one drive on your computer? If so, it would be interesting to know if perhaps paths on an external drive work -- that is a slightly different code path internally.
If anyone else is using High Sierra and paths with colons it would be interesting to know if they work.