I notice the article doesn't say anything about accuracy. This is not my area, but I think the _other_ hacky way to try to do spectroscopy with a phone is with a diffraction grating (and maybe a box with a slit in it). Diffraction gratings are cheap, probably not so different from a specially-printed reference card. If you have a choice, which is better?

diffraction grating wouldnt give you a controlled lighting environment (illuminant). they seem to handle that issue here by using a known spectral reference chart which might let them handle any normal lighting environment.

The actual article does say quite a bit about accuracy.