Probably not to achieve a level of accuracy better than just following an on screen calibration wizard. I say this having done studio photography where we used

Probably not to achieve a level of accuracy better than just following an on screen calibration wizard. I say this having done studio photography where we used a GretagMacbeth Color Checker (http://www.rmimaging.com/information/colorchecker.html http://www.rmimaging.com/information/colorchecker.html) to calibrate the camera back (PhaseOne P25+ on a Hasselblad H2) beforehand. The colors are already pretty accurate straight out of the camera but when you're trying to match specific colors between the print and real life objects, ever little bit counts. I wouldn't expect a mobile phone camera module to have nearly the same level of quality control as a medium format digital back either. The other reason (and this is probably more realistic, since most people don't do the full end to end calibration) is that the mobile phone camera would pick up the ambient light as well as what you were trying to measure. Colorimeters sit right on the screen (on CRTs they used to suction cup on) to block out any ambient light.