Matching the appearance of existing paint / colored surfaces is difficult.
You need data on the variation of colour, intensity and reflectance as a function of viewing and illumination angles.
Not something a mobile device can do.
The device described in this paper can:
(note: I am not affiliated; just out of interest)
If you have an app, and a standard color card[1]. then the processing to normalise the image (and therefore the colours) should be doable on a mobile device.
You need data on the variation of colour, intensity and reflectance as a function of viewing and illumination angles. Not something a mobile device can do.
The device described in this paper can: (note: I am not affiliated; just out of interest)
http://www.pro-lite.co.uk/File/RAD%20Imaging%20Sphere%20Pape...
[Edit: spelling]