Darkening of wood (ie. grey color) is a sign of water damage. Teak is more resistant than most but still discolors due to the minerals and chemicals in the water. This can usually be reversed with a bleach treatment. Oxalic acid is the wood workers choice but Laundry bleach will do. Do a test first using full strength. The natural warmth of teak can be reintroduced with a treatment of Wood Elixir which will also seal the wood to protect against future water damage.