How can I generate a color-correct h264/MP4 from a DPX sequence using ffmpeg?
I followed up on the yuv420p(tv, bt709) portion of the encoding and discovered that this refers to the Rec. 709 colorspace for HDTV. In ffmpeg, colorspace is defined by the colormatrix filter. However, you must convert from some colorspace. According to an ffmpeg mailing list conversation, the default colorspace is assumed to be bt601:


  ISTR that if you give ffmpeg RGB it assumes its full range, it also by
  default converts using 601 matrices. Although it has a number of
  colourspaces defined which can be accessed in the libs using the API,
  ffmpeg itself makes no use of them (Although I have a nagging feeling
  that that may not be completely true anymore either).


So the final ffmpeg command that worked was:

ffmpeg -y -start_number 0101 -i \\path\to\filename.%04d.dpx
-vf colormatrix=bt601:bt709 -pix_fmt yuv420p sample.mp4


The color is now correct, but there is no discernible difference in the output from ffprobe or MediaInfo. (In other words, the colorspace of the ffmpeg-generated file does not appear in the output.)