The key seems to be the difference between a colorimeter and a spectrophotometer. As you're using a smartphone camera (RGB sensors) you'd be simulating (seeming to work like) a spectrophotometer but emulating (working like) a colorimeter. I’ve used both kinds of devices. A colorimeter is perfect for things like monitor color calibration (emitters are RGB as well) but if you want the ink you’re printing to exactly match a sample in the real world you need a spectrophotometer (the latter’s absurdly more expensive than the former; the one I used belonged to a client)