I think the darker one is a neutral density filter. I believe the I1 camera has an aperture locked at f2.8. So, to get a correct exposure, the camera will vary the ISO and shutter speed. In some cases (maybe rare) the camera won't be able to get a shutter speed high enough and an lSO low enough to avoid blowing out some highlights in your picture. You should try both on a bright sunny day and see which you prefer. I will probably use the clear glass filter 90% of the time, but on some bright days, the ND filter may help balance the exposure some. I would recommend you ALWAYS have at least one filter on the front just to protect the front element of the lens. I have broken a few filters on my DSLR lenses and gladly paid the money to replace the filter knowing that it was much cheaper than replacing the front element of the lens . . . Being modular, I am sure you would need to buy a whole new camera if you even scratched the front element on the I1 camera.
The filter has threads on the outside. As in it screws over the mount, not on the inside like most filters. Normal 37mm filters will not work. Plus the filters included are only 3 grams. The commercial ones I have seen from Tiffen are 9 grams. Big difference!