I've had 2 male panthers do close to the same thing; one started @ ~11 months old and lasted ~4 months, the other was younger and started losing color @~7 months old and lasted ~4-5 months. Both lizards looked very promising before tuning dull and both became very nice adults and still are today. So, I'm wondering where you got your male?
If you look at my photo albums the males that went through dull colors are:
Rojo and Yaqui.