Default saturation for tweak is 1.0 . This does nothing to the clip. Values above 1 increase the saturation, values below 1 decrease the saturation as per the instructions
http://avisynth.org/mediawiki/Tweak
So if 2.0 is too much, try something in between like 1.5
You can use avspmod to view changes (push f5). I think there are sliders for tweak as well