I did some experimenting with my last roll of film. My camera lenses have Hoya UV filters on them. I wondered what would happen if I colored on the outside of the filter with different colored dry erase markers. I didn't notice a drastic change between different colored markers, but below are the results of marker versus no marker. The results resemble smearing Vaseline on the lens, but they are a little more subtle. If my memory serves me correctly, the top image was treated with blue and purple dry erase markers.