Water from your other tank isn't enough to cycle your tank but could have added some
ammonia,
nitrite, or even
nitrate without any good bacteria to eat all of it. Cardinal tetras are not a hardy fish and like to have a fully cycled tank in order to do well. Either
get a test kit and test your water or take it to your pet store and let them do it (though if they use test strips then you are better off with the liquid kit). What are your ammonia, nitrite, and nitrate levels at? Also what is your
pH? How long did you let the tetras sit in the bag in your tank before you transfered them and did you put some of your water into the bag over time to let them get used to your water? Generally fish lose their color because of stress, and just the trip from the pet store to your house could be enough to stress them out even with a fully cycled tank. If you tank water is within normal levels then it could just be travel stress and all the need is time, but my first guess is water quality problems really. If your Angel tank is fully cycled and you have a hang on back filter that's been in a while, you could cut a piece of that off and stick it into the filter on the tetra tank. That will help seed the bio filter with good bacteria. Another thing you can do is take a handful of the gravel from your Angel tank and put it into some pantyhose and hang that into the tank to help seed the tetra tank with the good bacteria. Let us know your water levels and we can go from there.