Also you might want to either signal the SD colorspace with the appropriate bitstream flags for NTSC content
Code:
--colorprim smpte170m --transfer smpte170m --colormatrix smpte170m
or convert the colorspace to HD specs with colormatrix mode "Rec.601->Rec.709".
https://avisynth.org.ru/docs/english...olormatrix.htm Since it appears you are upscaling to HD resolution. Some DVD's are encoded at Rec.709, there are instructions in the link to see how you can find out which colorspace is used.