A green spotted puffer can change the colour of its belly; usually form a bright white to a patchy dark/black colour.
This is generally down to the mood it’s in,
Usually when I’m not there and it’s either in its cave or just chilling on the ground the belly goes a dark colour
But as soon as I enter the room they are like little helicopters, wings all out, flying around, starring at me and
There bellies go a pure white.
Just make sure they are not bullied, have hiding spaced like caves (opening facing you so they can see you)
And ideally make the tank interesting, they love to explore looking for food, so use plenty of plants like vallis
Or tall fake ones / bogwood or ornaments to do so, feed them live bloodworm for extra interest and show your presence,
That will keep them happy.
provinding ware quality is good they should be ok
Also was the salinity the same in your tank as where they were previously in.
hope this helps.