Most fish, not just puffers, usually have their colors fade if they are just awakening in the morning, frightened, tired, asleep, sick, or even when happy but stuffed with food after a big meal. If yellow>>pale yellow/grayish yellow. If blue>> dull blue/ grey, etc. Some species make more profound changes than others.
It's not a problem if it happens and then normal colors return in the course of the day and the fish is behaving normally. After you have the fish awhile you'll notice its moods/tendencies, and you'll notice when something seems unusual and a possible cause for concern.