Cassini has many instruments. The "traditional" camera is black and white, but it is of super high quality.
However, the color in these pictures are not "photoshoped" in, from an artistic point.
It also has a infrared and Visible mapping spectrophotometer. This is where the color data comes from. If it was to be done as a picture, it would look horrible, using the spectrophotometer.
And I believe this is also the same method the Hubble Space Telescope uses.
The super high quality B&W for resolution and detail. The color, it is more important to get it in a format not like that of a picture, to analyze it properly from a scientific standpoint; but the color data can then easily be processed into the picture.
And your mobilephonecamera might got more mexapixels, but megapixels are not created equally, and I guanrtee it does not compare to its. Plus, as mentioned earlier it uses an entirely different system for taking pictures.