hwarang:
I've read that a mantis's color can fade if they don't have a dark enough burrow. Typically they don't actually make their burrows unless an adequate one isn't provided. What you should do is get a black piece of pvc, cap one end(or put an elbow and allow for a back door), and put an elbow on the other. wedge it under your LR in the tank for realism. It should take to the manmade burrow quite quickly.
If his color isn't improving you may want to check the iodide levels in the tank since it promotes molting ( too much can be toxic) and if it's too low he won't molt enough or too high he'll molt too much and maybe(not sure) that would effect his coloration.
Also, what have you been feeding him?