Colour sensors have 1/4 the resolution of a B&W sensor with the same number of elements (1R, 2G, 1B sub-pixel per pixel). If you know you are going somewhere that is uniformly grey you get more data from the higher pixel count. Also, an RGB sensor doesn't actually tell you anything about the colour of an object, it just looks like the colour of the object to a human eye. To know the actual colour of something you need to use a spectrometer to measure how much of each wavelength of light is being reflected.

tl;dr RGB colour cameras are pretty but don't help with science