well birds have better colour vision then humans we have three dimensional colour vision and they have four plus the UV.
Regards the preference to poop on your silver car, Stab in the dark here..but...is it because birds see the paler cars as a nice sitting spot because in the summer they get less hot but allow them to still catch the rays and because of this even though they might never sit on your silver car, local birds poop on the 'more attractive' silver cars just to mark it to say this is my fly zone stay out?? Or are the paler cars more attractive because they reflect plumage better like a mirror, or by reflection make them look bigger and make it look like two birds instead of one therefore less attractive to be attacked..
Or is there a big tree over you car
Or a bird feeder in the neighbours garden...lol