How to write a program for determining the intensity of the same color on android and convert the color into a concentration value?

Question

Image processing

The essence of the project is: To help the client choose a cassette. A client came with a bottle of water, our consultant conducted an express analysis using test strips and, based on this analysis, recommended a cassette.
In my understanding, the program should determine the intensity of the same color. The more intense the color, the higher the concentration of the substance, which determines the strip and digitize the color into a concentration value, for example, pale purple color - magnesium concentration is less than 5 mg / l, bright - violet - more than 20 mg/l. Therefore, the program must have some kind of internal calibration so that it can read and compare it with the internal color base and translate it into numbers.
How to write on IOS or Android? Are there similar sources on this topic? Need help!! Thanks in advance!!