Rolex seems to use a very dark shade of gold for some current  YG watches, e.g.  the Sub and the GMT, but a much lighter shade for others, e.g. the DD40. On their site they don't seem to acknowledge this...all the watches are described simply as 18 ct yellow gold.  I'm sure there must be some threads or sites about this, but I can't find any. Can anyone please point me in  the right direction?