There may be some color that will come back later but I have also seen some fish whose fins come back cellophane (clear) and stay that way. They always stay very fragile if they do not get their color back so I hope they get colored, but it will take a while as the first thing coming back will be the membrane and it is always clear.
Be patient as this is a good sign. Just give it time. Betta fins take a long time to heal and need clean water and patience. Patience, patience, patience. He will heal but it does take time. The thing that this tells me is that the meds you are using are the right ones. Just be careful not to do rough water changes or let him get too wild as the tissue is very fragile and can be knocked off at this point and then it has to start over with the healing process. So this is why I usually try to use the Fish Protector as it keeps them calmed down. If you do not have anything else you could try some Stress Coat or something with some aloe in the conditioner like Novaqua+ it will help keep him quieter if he is a nervous type betta.
The old formula never did but some of the meds have started to change their formulas so they could be putting melachite green or some other product in it now to help with the healing and it would turn the water green. I would not worry about it unless it has cautions for the tetras. It shouldn't though.
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