Hello,
many Leica threads are very tight and can only be loosened by special friction grip clamps which have exactly the correct diameter and even with this you have to use a lot of force. If you use a pair of tongs, you'll damage the surface. I thing this also so with the little round window of the range finder. Normally there was no colour glas installed, but you could mount it later. Perhaps you can try to glue a piece of a transparent coloured plastic film in front of the window, perhaps a little piece of a multigrade filter.