App for video mp4

I installed mpv but this is not good to manage, the elements for stop, start and slider are to small.

Did you try with Celluloid (formerly GNOME MPV) ?

I did, but the picture is lookind somewhat blue where it should be white.

Not sure if you saw my demo here:

That was when I was still on amber. Don’t know if it’s better with byzantium; I haven’t tested.

VLC flatpak was pretty good on amber: VLC (flatpak version) on L5

how about select ‘Settings’ -> ‘Display’ -> set ‘Scale’ to 100% -> ‘Apply’ ?