First thing I might do is test the water to see that ammonia=0 nitrites=0 and nitrAtes were 30 or below. Assuming these were in order ., I might consider that bright lights and gravel sometimes will cause fish colors to fade. Also possible that temp has become too warm or too cool which will sometimes happen during seasonal changes. Might check thermometer to see that all is normal. Other times dominance of one fish will cause other sub dominant fish to change to lighter color so as not to attract the attention of the dominant fish who may be courting a female in the tank. There are also foods out there that will help bring out the colors in fish. All fish should receive a diet of various foods to present their best colors. Hope some of this helps.