It looks like you are referring to two different color schemes: one WA scheme with white, WA blue and WA green, and the other with G45, B30 and G20.
For the Western Approaches (WA) scheme, you COULD use plain white, or you could use a very light grey that would be off-white. I would recommend a 50/50 mix of Humbrol 34 white and 147 Matt Light Grey.
For the WA Blue, I suggest Humbrol 65 Matt Light Blue. If you are willing to mix, I suggest one part Humbrol 89 Matt Middle Blue to three parts of 65.
For the WA Green, I suggest Humbrol 90 Matt Beige Green.
For the generic colors:
Steel decks: MS.2 (RN 05): Humbol 106 Matt Ocean Grey
Wood decks: Teak (C 01): Humbrol 103 Matt Cream or mix 121 Pale Stone with white, 50/50.
Underwater hull: RN Antifouling Red (RN 19): Humbrol 73 Matt wine
For the other set of colors (Light Admiralty scheme)
G45 ( RN 14 ): Humbrol 121 Matt Pale Stone
B30 Blue Green ( RN 21) Humbrol 65 Matt Aircraft Grey
G 20 Medium Grey Green (RN 25 ): Humbrol 27 Matt Sea Grey
Semtex ( Light Egg Blue ) ( M 16 ): Humbrol 28 Camouflage Grey
I suggest these as close matches; I am not claiming perfection. But I feel the most important factor is how the colors work with each other. I think these will be good for you.
Good luck,
Rick