Hi gurus

I have some old converted video from VHS which are somewhat defective in colour, contrast & not clear. In filtering options of Avidemux it appears to be able to fix to some degree but have not found the most effective way. I have 2 questions
  1. What are tbe some of the most effective filtering options to use for these defective. There are quite a wide range of options to choose from.
  2. In using the filtering to grey to avoid the colour defect, the preview shows the video in perfect grey, but on saving the result is the same as orginal without any changes. Why & how to fix?