There are a variety of illnesses that can cause a tree frog to become discolored. Red-Leg, a common disease in pet tree frogs, causes the skin near the frog's legs to turn reddish. Sometimes something as simple as lack of humidity, stress, or a poor diet can cause a frog to have darkened skin.  If your frog's skin is dry in addition to being discolored, this can be caused by a combination of stress and dehydration. However, some tree frogs naturally have a bit of red pigment on their legs. Keep track of your frog's coloring so that you can tell when it changes. If your frog is discolored, you should have it seen by a veterinarian. Lethargy can be a key symptom of a variety of illnesses that occur in tree frogs. They include Red-Leg, MBD, edema, and others. Look for a lack of activity, the frog's jaw to hang open, and its legs to be sprawled out.  Red-Leg is often fatal — there is no known cure for it. MBD is usually curable if it is treated right away. MBD usually makes the frog stop eating and jumping. This is due to improper feeding, which causes a lack of vitamin D and calcium. Edema is when the frog swells up and becomes lethargic because it is so bloated. If your frog is lethargic and not bloated, then it does not have edema. If your frog has swollen up, it is most likely caused by edema. There are actually two types of edema. One is a bacterial infection and the other is from kidney failure due to vitamin toxicity. If it is the bacterial kind, you should soak your frog in clean spring water for a few hours to clear out the infection. The frog should be alright after that. Unfortunately, if your frog has edema caused by kidney failure, it will most likely die from the condition.  Since bacterial edema can be cleared up relatively easily at home and the one caused by kidney failure is always fatal, it is up to you whether to take your frog in to be seen by a vet. To prevent edema, clean your frog's cage weekly and change its water frequently. Dying from kidney failure can be a slow, painful process. If your frog has edema caused by kidney failure, humane euthanasia performed by your vet is an option. Changes in your frog's appetite can signal a problem with their digestive system or an illness that does not allow them to catch their prey. For example, lack of appetite is a symptom of MBD. This disease weakens the bones of your frog and decreases its energy so much that it can't chase down its prey. MBD is often caused by improper feeding. Trouble with eating could also be the result of a parasite.  If you have more than one frog housed in the terrarium, some frogs could be getting more food than others. You should also make sure you are feeding your frogs small crickets, because medium and large ones are too big for a tree frog to swallow. Your frog may be ill if it is awake during the day. Lack of sleep can occur when the frog is stressed out, such as right after you bring it home. However, it can also be a symptom of diseases that make the animal uncomfortable, such as edema.
Check for discoloration. Assess your frog's energy level. Look for bloating. Consider trouble with eating. Pay attention to changes in your frog's sleeping habits.