Try the khaki method Steve

The thing with human perception of colour, is that it is unfortunately shaped for life by what we learn as children, mainly from kiddies' books.
So we all grow up knowing that sheep are white - except of course they're not. Look at any sheep in any field - they are somewhere on a cream - brown spectrum. Similarly, the sea is not blue - more often it is somewhere between green and brown. Grass in rarely green, but has a vast rage of shades, and even the freshest grass tends towards yellow on the spectrum.
And tree trunks are not brown. Look at most tree trunks, they have a lot of green in them - unsurprisingly really, because they're just big plants.
So old timber, even when it's bleached to almost white by age and the elements, still often has a slightly greenish tinge to it - but very rarely brown. Hence the khaki.