Color changes happen in Betta with a marble gene. It can happen when they're young; it can happen when they're near a year old. Has nothing to do with whether they are sexually mature and wishing to breed. JME.
Your betta may be losing color due to stress, old age, injury, and illness. Bettas can also lose color naturally, especially if they have the marble gene. ... you can improve your bettas color by making the water quality better, feeding them foods that promote vibrant colors as said in the above comment, and not overstocking your tank. A sick betta fish does not eat or eats reluctantly and may spit out his food. When it looks pale or its colors get dull or turn grey, it is a sure sign of a betta fish sick. The body may have open sores, white cottony patches, red and white spots. One or both the eyes may be swollen or protruding.