Thank you, I will put this info on the manual.
To understand accurately:

Source          YUV color Range

MPEG           JPEG                                            ==> Flat Colors

MPEG           MPEG                                          ==> Hard Clipping

MPEG          JPEG + Color Space (to JPEG)   ==> Color OK.


Did you use native color space plugin or F_colorspace?

I found (understandably) that importing MPEG into JPEG colour space leaves it rather flat even after my best efforts. I also found that yes, if I set Cin to MPEG it hard clips, so the waveform monitor 100% is ususally a straight line. However, if I leave Cin set to JPEG and import MPEG clips, then convert it to JPEG using Colour Space, the flatness disappears and so does the hard clipping.

Edit: I forgot to say the HC-X1500 exposure is normally set using 95% zebras, so the waveform monitor should never hit 100% with a raw clip.

Just thought I would mention it in case it helps.

Thanks again for your input.

I can confirm that formats using bgra seem to match the compositor when using JPEG as the colour model, I can then convert to m4v using Handbrake, and upload that file to YouTube.

Colours seem reasonably consistent in YouTube.

So the issue appears to be in how JPEG colour model YUV files are written by Cinelerra, as the files converted from RGB to YUV by Handbrake are displayed correctly. Is this a separate issue to the clipping of out-of-range values when in MPEG color model?

It is good webm-gbrp works also for you.
I don't know why You and me are seeing different results in some cases.
May be it depends on my old software versions and my screen.
To compare with te Compositor I am using both VLC and ffplay.
The info about.
- VLC media player version 2.2.2 Weatherwax
- ffplay version 2.8.6
- My screen is calibrated on sRGB with an external probe.

However I can see that if we use a Rendering format with "Pixels: gbrp" (see Render #2 of my previous post) or "Pixels: rgb" and using ffmpeg by command line to convert the CinelerraGG output to h264 yuv420p/yuv422p/yuv444p, the conversion seems good for you and for me. It might be a good but "undesiderable" workaround (strategy).

Another good Render is...

- Render #4:
  File Format: FFMPEG | mkv
  Video Preset
    Compression: user_ffv1.mkv (or ffvhuff)
    Bitrate: 0
    Quality: -1
    Pixels: bgra

Converted by ffmpeg command line to...

ffmpeg -i input.mkv output.mp4

(default pix_fmt will be yuv444p)

or for yuv420p ...

ffmpeg -i input.mkv -pix_fmt yuv420p output.mp4

My ideas are on a dead line, now. Sorry.

For me, a webm video using your settings looks very similar to the compositor, however the mp4 video does not. The mp4 video suffers from the same dark/more contrast difference as before. Webm is very slow to render!

the Render below is playable in VLC if it has been created from CinelerraGG_20201031,
NOT if it has been created from CinelerraGG_20211231. Is it just my problem?

Works fine for me using CinelerraGG_20211231 (with the usual too dark/too much contrast).

Also, I also tested CinGG-20201031-x86_64-older_distros.AppImage, but didn't see any different results.

Below the Render settings that seem to match good VLC vs Compositor
Could you do a test with these Render, please?

- Render #1:
  File Format: FFMPEG | mp4
  Video Preset
    Compression: h264.mp4
    Bitrate: 0
    Quality: -1
    Pixels: yuv422p (ffprobe will show it is changed to yuvj422p)
            yuvj422p

- Render #2:
  File Format: FFMPEG | webm
  Video Preset
    Compression: webm.webm
    Bitrate: 0
    Quality: -1
    Pixels: gbrp

Settings used in CinelerraGG:

- Preferences:
  - Playback A TAB
    - Video Out section
        Play everyframe = unchecked
        Video Driver: X11
                      Use direct x11 render if possible = checked
  - Appearance TAB
    - Color section
        YUV color space: BT709
        YUV color range: JPEG


- Format:
  - Video section
      Frame rate: ---
      Width: ---
      Height: ---
      Color model: RGBA-8bit

It is strange,
the Render below is playable in VLC if it has been created from CinelerraGG_20201031,
NOT if it has been created from CinelerraGG_20211231. Is it just my problem?

- Render #3:
  File Format: FFMPEG | mp4
  Video Preset
    Compression: h264.mp4
    Bitrate: 0
    Quality: -1
    Pixels: yuv444p (ffprobe will show it is changed to yuvj444p)
            yuvj444p

Thanks!

 st: 0 edit list: 1 Missing key frame while searching for timestamp: 3600
 st: 0 edit list 1 Cannot find an index entry before timestamp: 3600.
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '18-surfer-wide.MP4':
  Metadata:
    major_brand     : mp42
    minor_version   : 1
    compatible_brands: mp42avc1
    creation_time   : 2021-12-23T17:01:38.000000Z
  Duration: 00:00:22.56, start: 0.000000, bitrate: 92715 kb/s
  Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuvj420p(pc, bt709), 3840x2160 , 92496 kb/s, 25 fps, 25 tbr, 90k tbn, 50 tbc (default)
    Metadata:
      creation_time   : 2021-12-23T17:01:38.000000Z
      vendor_id       : 
  Stream #0:1(und): Audio: aac (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 125 kb/s (default)
    Metadata:
      creation_time   : 2021-12-23T17:01:38.000000Z
      vendor_id       : 

Your outputs are as mine. Good but, until now, I can not reproduce this output inside Cinelerra-GG and rendering in h264-yuv444p: it always render to yuvj444p and it looks a bit more dark. (And I write color_range, color_primaries, color_space, movflags=+write_colr in the Render codec)