The primary cause for color changes in a Betta is a sudden change in water paramters such as a rapid fluctuation in water temperature and ph.
Another reason for the color change could be genetics and/or old age. Note that the average life span for a Betta is 2 years and they are usually available for sale when they are 1 year old.
What I suggest you do is make sure the Betta' basic needs are being met such as a steady water temperature of 80f degrees, slow moving or no movement in the water and clean water which can be provided by doing 100% water changes every 3-4 days if the fish is in a bowl or 50% water changes if the fish is in a tank 2 gallons or bigger.
As a preventative measure you can add some aquarium salt at a ratio of 1 teaspoon per every gallon of water and add a product to the water called Betta Fix, see link below for a description of the product.
It would also be helpful to feed the fish a varied diet that includes flake food, frozen food and freeze dried food.
I don't think there's cause to be alarmed by the color change but I certainly can understand your concern.
Best wishes and please let me know if you have any questions.
http://reviews.petco.com/3554/10673/reviews.htm
.
YOUR TROPICAL FISH RESOURCE
OVER 20 YEARS EXPERIENCE W/TROPICAL FISH---BREEDING, MAINTAINING, CURING