Hi Roy you could try these mixes,Ivory Black, Titanium White,Yellow Ochre,Viridian Green or some similar green plus a touch of Raw Umber or Sepia to tone it down.Hope this is of some help.
Hi Roy, it's worth highlighting that there's no one best mix for any colour. There are usually multiple ways to mix certain colours and dull ones like those we're often called on to paint in particular could have a dozen or more different possible routes! Sometimes the choice merely comes down to using what you have - which might be slightly, or very, different to other people's selections of paints.