[FFmpeg-user] [prores] Why a little color difference by converting from ProRes 422 to MPEG-2

[linxs]

Hi,

I get a .mov file and its video format is "ProRes 422, 24 fps, 1920x1080,
BT.709".
Using ffmpeg to convert it to MPEG-2:
ffmpeg -i my.mov -c:v mpeg2video -b:v 30M -pix_fmt yuv420p my.m2v

I compared them by previewing "my.mov" and "my.m2v" in Edius and I found
there is a little color difference:
"my.mov" seems to be a small little brighter than "my.m2v" and the color in
"my.m2v" seems to be a little more saturate.

I used "ffmpeg -i <input> -c:v rawvideo -pix_fmt yuv420p <output in yuv>"
to check the yuv ouput of both mov/m2v files.
I am sured that the yuv output of both are almost the same.

Why is a little color diffence between the two files?

Thanks for help!

linxsy