You can if all your stix are measuring is your dietary fat.
Remember, ketostix measure dietary fat burned as well as bodyfat and don't make a distinction between the two at all.
So, if you're eating tons of fat, enough to take in more calories than you need, your stix will show purple and you could still be gaining weight.
Granted, it's difficult to take in that many calories in pure fat, but you can still gain weight if there's too much, especially if it's accompanied by enough protein to get insulin release.
Fawn