The AIRFIX painting instructions are indeed wrong, but so is the statement on the PTBOATWORLD site that the colors were reversed. <<The colors of the scheme were reversed. What was supposed to be light gray was instructed to be dark gray and what was supposed to be dark gray was supposed to be painted light gray instead of the other way around.>> This would make the areas that are supposed to be white a dark gray.
Photos of MTB 385 on trials show her in Admiralty Basic. The correct colors are white, G45 (a light gray) and B15 (a dark blue-gray). You can get the correct paint from WEM www.whiteensignmodels.com
Here is the correct pattern from C.B. 3098 The Camouflage of Ships at Sea 1943:
http://i73.photobucket.com/albums/i204/alross2/vosper.jpg
While this depicts a 70' Vosper, the layout applies to the 73' Vospers as well. If you can access John Lambert and Les Brown's ALLIED TORPEDO BOATS, take a look at Christian Sheppard-Capurro's MTB 386 on page 72.
Al Ross