I'd need to see the colours painted onto something Andy. As mentioned, the OD might do the trick as an 'Equivalent Colour', normally a Du Pont paint used by Grumman. The Sky should be OK, as Grumman used this and the equivalent Du Pont paint 'Sea Grey' in one of their plants, which looked between Sky and a blueish grey. For the Slate Grey, try adding a touch of the OD or a dark green to the mix, as the actual colour was a greenish grey. At certain angles, depending on the light, the two tended to almost blend into one colour.
The attached pic is a bit bright, but might give you an idea - I just mixed the paints until they looked more or less right, allowing for the darkening effect of the following clear coats, which haven't been applied in this pic.