Did you follow the washing instructions Sally? Maybe you didn't do anything wrong, maybe the quality was just poor. I've just bought a pair of dark (called black, seem not quite black to me) cropped jeans to wear with my new boots. They said to wash them separately beforehand as otherwise the dye may rub off on other lighter clothing. They seem still a good colour but not as crisp after that, haven't worn them yet. I also avoid the dryer for jeans. I dried them on a rack inside since we have had so much rain as you know. Hope you were OK in Cyclone Cook. We were good, it seemed to mostly miss Auckland.
I am not totallysure about the quality of all of the stuff at Just Jeans, although Lee Riders is a famous brand that I remember fondly, my older daughter had a great pair when she was a teenager ( now 26).