The process is:
export the Org mode file (say foo.org) to a tjp file (foo.tjp).
process the tjp file with tj3: `tj3 foo.tjp
view the resulting HTML (Plan.html) file with a browser (both Firefox and Chrome worked for me).
Print from the browser to PDF, but make sure to first open More settings and select the Background graphics (Chrome) or Print backgrounds (...
I'm no expert by any means but one partial solution is proposed here:
Basically they say to print the page from a web browser while visiting the
.html file generated by org-taskjuggler-export-process-and-open.
However, at least on Firefox the generated pdf skips the colors and ...