Color changes after video to single frame images back to video using ffmpeg
Solved the problem by setting colormatrix conversion using swscale lib. Added to my filter chain:

scale=in_color_matrix=bt601:out_color_matrix=bt709


and

-colorspace bt709


to the output.



References


ffmpeg scale filter
ffmpeg colormatrix filter
colorspace on ffmpeg wiki
question on video.stackexchange




Credits

Found the advice on VideoHelp forum reply from 2017, which in turn references the question on this video.stackexchange.  Thanks again, Gyan!