Lol I just posted part of this for someone else =P
Red tailed sharks can sometimes take up to a month or two in a tank ( with the right conditions ) before really becoming colorful and even more time to completely fill in. a happy healthy RTBS will be completely black ( excluding females that tend to have grey underbellys ), jet black so to say except for their blood red tail fin and the white tips that form on the other fins. In order for a RTBS or any FW shark in its family like a rainbow or a albino red tail to become happy enough to gain its full color it must be housed in a tank no smaller than 50g, it must be kept with compatible fish mates here is an excellent guide Freshwater Fish Compatibility Chart: Avoid Incompatible Species, the water parameters must keep steady, any ammonia levels will stress out your shark and cause it to lighten color, it must be fed appropriately( sharks like sinking pellet food only over time will they accept fish flakes ) if you follow these guidelines anyone that has a fw shark will have a brilliantly collored, exciting centerpiece for their community tank.
If i were to guess why yours is loosing color it may be unhappy with the tanksize, tankmates, or could just need some time to adjust.
A happy redtail will always be deep black and have a blood red tail regardless of lighting or time of day. its color depends on its happiness.the biggest thing i can say is make sure to have compatible tank mates, for the RTBS is very aggressive to certain species, the rubbing on the gravel and decor is called "flashing" and is a behavior of a very angry shark but could also be something serious like ick, get your water test and watch for small dots about the size of salt grains, dead giveaway of ick. hope this helped.