Like any wood product, Teak does require some upkeep. If left alone it will turn a silvery tone like you may see on park benches, although many of these benches around the world are 80 plus years old. 

There are many products on the market today for "Brightening" and "Cleaning" Teak. Most of these products are harmful to the Teak, quite harmful to the environment and not too terribly healthy for the user who must handle and breathe these products. 

Teak after time will turn dark, mainly from all the dirt that has been ground into it. For a light cleaning of teak, simple wash it with lots of water and a soft scrub brush, you'll be amazed at how much dirt it will release and immediately start to brighten. Stainless Steel wool works quite well also. For a more stubborn job use a light solution of soap and water. If your boat is in the water, a soap that is enviromentally friendly would be recommended. For a really tough job, a solution of bleach and water will work well mixed at a 50/50  blend. Should Teak have been neglected for some time, sanding may be required, but remember this will reduce its naturally non-skid properties and reduce its lifespan. 

After Teak has been cleaned, Teak oil can then be applied, this will give the Teak a lovely look and also aid the Teak in its naturally self-oiling properties which allow it to ward off mold, mildew and many germs. Teak oil can be applied with a rag or most any type of brush. It is recommended to wipe off with a dry rag upon completion. Remember always that Teak likes the sun, if you'd like it to blend or lighten, give it some fresh air and sun light.