Your best bet is a good quality Teak oil. West Marine, Starbrite, Amazon, Interlux are all good. The oil will protect and nourish the wood but will not prevent it from turning grey/silver. The grey/silver is mold growing in the pores of the wood. In order to keep it the rich wood colors it will need to be cleaned and brightened a few times per season. There are commercial grade cleaners and brighteners by all the manufacturers above but the are expensive and contain caustic materials that will damage the wood. We have found you get an equally clean and bright job by cleaning with with Ivory liquid dish soap and then wiping a 1/4 bleach to water will kill the mold and brighten the teak. Let dry for a day or so and re-oil. DO NOT use a stiff bristle brush with the grain you will create crevices in the wood and cause the mold to grow faster and damage the wood.
Also unless you are in the mood to constantly be maintaining the table and chairs do not use any type of sealer such as varnish, tung oil etc. The suns UV and the expansion contraction and the natural oily nature to the wood will cause the finish to crack and mold to grow in the cracks. A good varnished teak requires 1 to 3 reapplications of finish per season depending on climate. 1 up North 3 in the tropics to look good with a complete strip and refinish about every 5th year or so.
Varnished table 10 + coats. Natural deck oiled about two times per season.
