New answers tagged color-correction
What you're describing is color grading / color correction and yes, it's perfectly acceptable, almost mandatory for a good presentation. Usually it's done scene by scene, but it's not at all unusual to correct within a single shot, or even different areas within a shot. To do it well you should explore a dedicated grading tool like DaVinci Resolve Lite.
IDK how well this would work. Prob. not at all for vids with differing GOPs, but you could try using something to mirror your keystrokes to multiple windows. Autohotkey can do that on Windows. On GNU/Linux, Terminator can send keystrokes to multiple gnome-terminal tabs at once, so you could control mplayer that way (since it reads the tty as well as ...
Top 50 recent answers are included