Unprotected exterior wood is prone to damage and decay. It is important to maintain exterior wood by cleaning it routinely and sealing/staining it to keep it looking its best.
When the situation arises that a board has been damaged due to the elements or has rotting occur, it will need to be replaced. When replacing a deck board it is important to know what you are getting in to. When you put a brand new board next to an older board, the two will never match. The only way to aesthetically get the boards to match is to use a solid stain to cover up the boards. When using a transparent stain or a semi-transparent stain, once the new board is installed it will be very obvious that a new board was installed. Of course in some situations there is not an option to prolong replacing boards, however it is important to know what your deck will look like once this service is completed and to know your options for maintenance.
The picture below shows old boards (more reddish), next to boards that were replaced about a year prior to this picture (gray boards) and then new boards that were just replaced. With this deck, the only way to make the surface match is to properly prepare the wood and then to apply a solid stain. The gray boards were left un-stained or finished (turned gray due to being un-protected and exposed to the sunlight).