Your aquarium light serves three separate purposes.
Firstly, and most obvious is this.
Your Aquarium Light or Aquarium Lighting System is there so that you can enjoy looking at your fish, and their behavior and interaction with each other.
Secondly, the live plants in your tank require a certain amount of light to photosynthesize.
The process of photosynthesis is when plants use light to convert certain chemicals into other substances they can use for food.
Thirdly, your aquarium lights give your fish the illusion of night and day.
Fish don't really sleep, but they do require a certain amount of rest.
In nature this rest will occur when it gets dark.
Although you can have your lights on all the time, and your fish will get used to it, it is important not to in the case of plants. Your plants need 2 cycles, Light and dark to survive.
Your best bet would be to buy a timer switch that will turn your light on and off in 12 hour cycles.
These would be available at your local hardware store.
I personally prefer using a fluorescent globe system that you can buy at your local pet store.
To date I have only been able to find three different colors for these being:
White light seems to work best in big heavily planted tanks, and may take on a slightly greenish tint if the light shines through some floating plants.
Red fluorescent lights are a little more subdued and less bright but can be used to great effect if you like seeing the caustic reflections of the water ripples on the wall.
Both White and Red fluorescent globes can be used in a tropical fish tank.
Blue fluorescent lights are generally used for low light marine aquariums since the spectrum of these lights don't do much for plant photosynthesis, but does make it feel like you are really 5 meters under the water swimming over a reef.
Blue Light also seems to bring out the colour of your tropical fish, but make sure to combine it with either red or white light to ensure your plants grow properly.