I have the original GR1 and I tape a gel filter on the back of the lens. Works fine, but I am committed to the filter once I put film in. Not really a problem for me since I almost always use the same filter color. If you have a professional lighting house nearby, you can pick up a filter swatch book for free. They work fine as long as they aren't scratched up. I used the same filters in the gel holders on the rear of my wide angle lenses.