Another thing is that this sensor will not work outdoors, because the UV will kill the plastic. You can improve it a bit by adding clear glass in front of the plastic parts. It will reduce the UV radiation, but the color of the plastic will still fade quite quickly with time.

And while you may be able to detect that there is a color, you will not be able to measure what color. Another thing is that a fish line is a thin line. You would prefer a colored film in front of the surface, i.e. a thin two-dimensional object.