ffmpeg output gray

>ffmpeg -framerate 24 -f image2 -i "%04d.png" -c:v libx264 -preset slow -crf 23 video_test.mkv
ffmpeg version N-80892-g2a8dadb Copyright (c) 2000-2016 the FFmpeg developers
  built with gcc 5.4.0 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-dxva2 --enable-libmfx --enable-nvenc --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-libzimg --enable-lzma --enable-decklink --enable-zlib
  libavutil      55. 28.100 / 55. 28.100
  libavcodec     57. 48.101 / 57. 48.101
  libavformat    57. 41.100 / 57. 41.100
  libavdevice    57.  0.102 / 57.  0.102
  libavfilter     6. 47.100 /  6. 47.100
  libswscale      4.  1.100 /  4.  1.100
  libswresample   2.  1.100 /  2.  1.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, image2, from '%04d.png':
  Duration: 00:00:01.08, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: png, pal8(pc), 3840x2160 [SAR 2835:2835 DAR 16:9], 24 fps, 24 tbr, 24 tbn, 24 tbc
No pixel format specified, yuv444p for H.264 encoding chosen.
Use -pix_fmt yuv420p for compatibility with outdated media players.
[libx264 @ 00000000001ec5e0] using SAR=1/1
[libx264 @ 00000000001ec5e0] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX XOP FMA4 LZCNT
[libx264 @ 00000000001ec5e0] profile High 4:4:4 Predictive, level 5.1, 4:4:4 8-bit
[libx264 @ 00000000001ec5e0] 264 - core 148 r2694 3b70645 - H.264/MPEG-4 AVC codec - Copyleft 2003-2016 - http://www.videolan.org/x264.html - options: cabac=1 ref=5 deblock=1:0:0 analyse=0x3:0x113 me=umh subme=8 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=4 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=2 b_bias=0 direct=3 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=24 scenecut=40 intra_refresh=0 rc_lookahead=50 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
[matroska @ 00000000001e7fe0] Using AVStream.codec to pass codec parameters to muxers is deprecated, use AVStream.codecpar instead.
Output #0, matroska, to 'video_test.mkv':
  Metadata:
    encoder         : Lavf57.41.100
    Stream #0:0: Video: h264 (libx264) (H264 / 0x34363248), yuv444p, 3840x2160 [SAR 1:1 DAR 16:9], q=-1--1, 24 fps, 1k tbn, 24 tbc
    Metadata:
      encoder         : Lavc57.48.101 libx264
    Side data:
      cpb: bitrate max/min/avg: 0/0/0 buffer size: 0 vbv_delay: -1
Stream mapping:
  Stream #0:0 -> #0:0 (png (native) -> h264 (libx264))
Press [q] to stop, [?] for help
frame=   26 fps=7.2 q=-1.0 Lsize=       9kB time=00:00:00.95 bitrate=  79.5kbits/s speed=0.264x
video:8kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 10.896616%
[libx264 @ 00000000001ec5e0] frame I:1     Avg QP: 9.41  size:  1936
[libx264 @ 00000000001ec5e0] frame P:7     Avg QP:10.14  size:   245
[libx264 @ 00000000001ec5e0] frame B:18    Avg QP:12.67  size:   237
[libx264 @ 00000000001ec5e0] consecutive B-frames:  7.7%  0.0%  0.0% 92.3%
[libx264 @ 00000000001ec5e0] mb I  I16..4:  0.0% 99.9%  0.0%
[libx264 @ 00000000001ec5e0] mb P  I16..4:  0.0%  0.0%  0.0%  P16..4:  0.0%  0.0%  0.0%  0.0%  0.0%    skip:100.0%
[libx264 @ 00000000001ec5e0] mb B  I16..4:  0.0%  0.0%  0.0%  B16..8:  0.0%  0.0%  0.0%  direct: 0.0%  skip:100.0%
[libx264 @ 00000000001ec5e0] 8x8 transform intra:99.9%
[libx264 @ 00000000001ec5e0] direct mvs  spatial:33.3% temporal:66.7%
[libx264 @ 00000000001ec5e0] coded y,u,v intra: 0.0% 0.0% 0.0% inter: 0.0% 0.0% 0.0%
[libx264 @ 00000000001ec5e0] i16 v,h,dc,p: 71% 21%  7%  0%
[libx264 @ 00000000001ec5e0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 91%  1%  8%  0%  0%  0%  0%  0%  0%
[libx264 @ 00000000001ec5e0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 33% 25% 42%  0%  0%  0%  0%  0%  0%
[libx264 @ 00000000001ec5e0] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 00000000001ec5e0] kb/s:58.41