Her tail first starts out pink, but then gets changed to sky blue because of the water fall. A little while after that, the girls' make a potion, Amy is the only one willing to drink the potion, making her 'pop' a tail, and letting them see that it has changed to the color of the potion.
Her tail first starts out vertically striped, like a rainbow, with some silver and gold in it. It then is changed by the waterfall to a soft yellow.
Kelsey's tail starts out Green, and never changes, because she did not have her tail, when the other two changed theirs.
They make the picture fuzzy every time Tessa is in a scene, but it is believed to be a yellow or a green.
Where to get the tails: Many people buy their tails from: Amazon, ebay, Mermagica and a few other places.