I just bought two bullfrog tadpoles from a person who breeds them in his outdoor pond in his yard. this is the same person i got my first bullfrog arya from over a year ago. One tadpole I have right now has a yellowish belly and the other has a white belly. they are both about 4" long and one is starting to morph. I added this because when i picked out arya as a tadpole i picked her because she had a white under-belly hoping it was a female and she did turn out to be a female. I do not know if it was just luck or if the color is slightly visable allowing sexing as tadpoles. none the less when my tads morph i will let you all know on this thread if my assumptions of the sex in the tadpoles are correct. But like you said Brian, coloration is not all ways reliable even for adults.