Currently the colours are generated from packet loss, which is based on the packet returns from traceroute. Red is 0% loss, blue is 100% loss from 10 packets. However, nodes with 100% loss are not shown (since traceroute cannot reach them to determine an address).
The script ndt2gif and associated files are available from sitka.triumf.ca/pub/ndt and sitka.triumf.ca/pub/ping. At this time the relevant HTML pages (such as this one) must be saved manually.
Andrew Daviel, TRIUMF