Wrong PGS subtitle colourspace

[Carl Eugen Hoyos]

(videolan ticket 14777)
The subtitle background is gray which is apparently wrong, it should be the same colour as the movie frame (black).

$ ffmpeg -i darkgraysubs.m2ts -filter_complex [0:0][0:2]overlay -qscale 2 -an out.avi
ffmpeg version N-72996-gd1050d9 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 4.7 (SUSE Linux)
  configuration: --enable-gpl
  libavutil      54. 27.100 / 54. 27.100
  libavcodec     56. 41.100 / 56. 41.100
  libavformat    56. 37.100 / 56. 37.100
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 17.100 /  5. 17.100
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.100 /  1.  2.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, mpegts, from 'darkgraysubs.m2ts':
  Duration: 01:57:18.53, start: 600.000000, bitrate: 0 kb/s
  Program 1
    Stream #0:0[0x1011]: Video: h264 (High) ([27][0][0][0] / 0x001B), yuv420p, 1280x720, 23.98 fps, 23.98 tbr, 90k tbn, 47.95 tbc
    Stream #0:1[0x1100](eng): Audio: ac3 (AC-3 / 0x332D4341), 48000 Hz, stereo, fltp, 96 kb/s
    Stream #0:2[0x1200](eng): Subtitle: hdmv_pgs_subtitle ([144][0][0][0] / 0x0090), 1280x720
Please use -q:a or -q:v, -qscale is ambiguous
Output #0, avi, to 'out.avi':
  Metadata:
    ISFT            : Lavf56.37.100
    Stream #0:0: Video: mpeg4 (FMP4 / 0x34504D46), yuv420p, 1280x720, q=2-31, 200 kb/s, 23.98 fps, 23.98 tbn, 23.98 tbc (default)
    Metadata:
      encoder         : Lavc56.41.100 mpeg4
Stream mapping:
  Stream #0:0 (h264) -> overlay:main
  Stream #0:2 (pgssub) -> overlay:overlay
  overlay -> Stream #0:0 (mpeg4)
Press [q] to stop, [?] for help
frame=  241 fps=180 q=2.0 Lsize=     394kB time=00:00:10.05 bitrate= 321.5kbits/s dup=0 drop=2
video:383kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 2.982323%