Why Does My Puffer's Belly Change Colors?
There are a few reasons a GSP's normally white underside may turn black or dark gray, all relating to stress. If notice a GSP's underside turning a dark color the first thing that should be done is check water parameters. If there is any ammonia or nitrite, an immediate water change of at least 50% is needed. Any trace of ammonia or nitrite can cause a scaleless fish like a puffer an enormous amount of unneeded stress. Continue with water changes as needed.
If the water parameters are good, then how the puffer is being housed may be the cause. If the tank is less than 30gallons per fish then it's definitely time to upgrade. If the tank size is appropriate, then more decor added tank to make it more interesting for your puffer may help. Puffers are very intelligent, inquisitive animals and need lots of interesting decor to keep them from getting bored. A good mixture of tall and short decor is vital for a good GSP habitat. Another cause of a stressed puffer could be an inappropriate amount of salt in the tank. A larger GSP in a freshwater or low end brackish tank can lead to many illinesses and a generally shorter life.
Once certain the tank is an appropriate home for a GSP the next thing to check for is illness. Be careful when identifying illnesses and before doing any sort of treatment get the opinion of someone with lots of experience treating GSP illnesses. White spots on the body does not always mean ich, GSPs have small spines that remain hidden until the puffer "puffs" these spines are seen as tiny spots. A sunken in belly usually means Intestinal parasites and should be treated immediately.
Some puffers are just naturally nervous of new things. If everything else is okay, he could just be frightened of something trivial like a loud noise. Puffers are also known to get stressed when small things like the lights turning or off, new decor being added, a water change, or someone they don't know walking past the tank. Turn the lights off for a few hours and see if he/she reverts back to the normal bright white coloration.
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