I think the only way to get something you are happy with is to practise on scrap material and compare the results with as many period photographs as you can get your hands on. The trouble with teak decking is that when dry it is actually very light coloured but as soon as it gets wet it goes very dark. Assuming you want to model it dry then I would start off with very light grey with a spot of beige or buff mixed in.
Despite models occasionally displaying varnished teak decking this is a pretty rare finish with it perhaps only occurring on such vessels as yachts and even then only in small areas. Even to this day teak decking on ships is simply cleaned with 'holy' stones so the finish is a very matt rough surface.
Interestingly of course most brand new ships do not use teak as a deck covering as it is A) Too much of an environmental concern and B) Very expensive to fit, maintain and repair. Most modern ships are now fitted with some form of resin poured deck covering. Quite why they insist on then trying to make it look like real wood with caulking lines I've never been able to work out!