Hi Nige,
A good question, For red on it's own I would normally suggest a shade of brown, and for white, on it's own a light to medium grey. for the 2 colours together then a brownish grey would seem to be the answer (clever these older folk

). Now Paynes grey is a blue grey and might be the right shade for upper surface RLM 74/75 scheme but Davey Grey is a light browney grey (you obviously wasn't listening at MK [youngsters, bah]

)
or you could try mixing you own browney grey, but artist oil paints can throw you a few fast balls, because of their make up. If you don't have or don't want the expense of obtaining Davey grey which, as mentioned at MK

is a good wash colour for Sky (remember now

You could simply mix black and white for a medium grey, which has been mentioned by Greg.
If you are using artist oils, over enamel paints, then you must protect the paints. Klear (Future) is a very good clear coat for this. Use pure turpintine (not Turps substitute) to thin the oils. The Wash shouldn't "defeat" the pre-shading, but should compliment it.
Looking forward to seeing your 190.
Mal