There is no direct correlation between any of the US Navy's WWII ship camouflage colors and the Fed_Std_595 system. Any chart which offers such a conversion is someone's personal opinion. And we all know about modelers opinions -- you ask 6 modelers their opinion on a subject and you will get at least a dozen different answers.
The Navy's WWII colors were based on the 1929 Munsell Color System standards. They were based on a purple/blue scheme. Early in the war paints were mixed at the yard (or on board ship) by adding so many units of Blue Tinting Medium paste to so many gallons of white paint vehicle. The colors of batches varied greatly. Later in the war the paint came pre-mixed and was more uniform. By very late in the war (i.e. summer, 1945) - the Navy terminated the use of the purple/blues in favor of neutral grays.
It is also important to point out that the 1929 Munsell Color reference is not consistent with today's Munsell color notation -- you cannot get to the correct color using todays Munsell system and the WWII color specification.
The Fed_Std_595 system was developed in the 1950s in an attempt to standardize the color specifications of not only the military, but also the civilian agencies (USPS, VA, NPS, Forest Service, etc.). Some similar colors were lumped together -- close enough for government work. Obsolete colors were not included in the specification, a'la the WWII naval blues.
Furthermore the Fed_Std system is not extensible. You cannot compare a color with a set of standard chips and say that the new color is one unit bluer and three units grayer than a standard. Again, practicioners of this are exercising their own personal opinion.
White Ensign is not the only point of sales for their paints. In the States you can order the paints from Randy Short at the shipcamouflage.com website you cited. Randy and John Snyder of WEM used to be partners in Snyder & Short. They developed the original paint chips which became the WEM Colourcoats. Colourcoat paints are also available through the Floating Drydock website and through Pacific Front Hobbies online store. You can buy them over the counter at AAA Hobbies in Marietta, GA and I believe from Colpar Hobbies in Aurora, CO. Theyre may be others.