I loaded a test clip from
https://kodi.wiki/view/Samples
Specificially, Nr. 16 under 4K UHD, this file:
https://mega.nz/#!hJdFEIBI!uSOjZtkkjIVYSfqD9aSfONf1yq__uQvlsf47pCtFvdQ
(The World in HDR)
When I play this file in various media players (MPC-HC, MPC-BE w/ LAV codec, VideoLan), this clip is very color-saturated and has a high contrast.
When I load the clip with Avidemux, the clip is dull, far less saturated and far less contrast. I also played around with output dynamic range settings (eg full, limited etc.). No dice. No matter what I do, the colourful clip looks entirely different in Avidemux, and then also when I save it.
(From what I remember, I also tried to load the test clip in VirtualDUB. There, the clip is also very de-saturated and dull, but VirtualDub crashed at some point. So whatever is changing the clip, it must not be related to Avidemux *per se*.)
Edit: If you load the clip in Avidemux, it is not simply desaturated. The colours are just very off. Watch the original clip, and then load in Avidemux and preview it!
Must be related to 10 bits color depth. Avidemux uses 8 bits YV12 (yuv420p with swapped U and V) internally. Avidemux, ffplay and mpv all show identical pale brownish tint when playing this sample on my system, only VLC applies necessary corrections.