The color change can be a number of things. Stress can bring it about as can illness or the fish simply not feling well. If you want to keep this fish for a long time and keep it healthy, wean it off live foods ASAP. Fresh water feeders are not healthy and can cause damage to internal organs if used for long periods of time. THey also lack the necessary nutrients that salt water predators need. Additionally, they can introduce pathogens into your system that can make the fish sick.
Try using a feeding stick, and wiggle some frozen foods in front of the lion. They often take this as live prey and will go for it. Small pieces of shrimp, scallop, whitefish, krill, silversides, talapia, etc work well. Be persistant. If you give in, the lion will never accept prepared foods. It's fine to withold food for a few days as well to get the lion really hungry. Forego your desire to see the fish hunt, in favor of keeping it alive.
