I'm having a problem with my 5D where the colours on manual video mode are washed out/grey, whereas on all the other settings the colours are much more vibrant and consistent between them. It is only on manual that the colours seem to shift. I've played with every setting I can imagine, and the white balance is the same for each setting, but I can't make sense what the issue is. I've made a video demonstrating the problem.
Are you capturing in log format? Such footage will have a much flatter apperance (unless a LUT is applied when viewing such footage; not sure if the 5D IV does that).
What are your camera and exposure settings used in the different modes?
Does this also happen when you shoot stills in Manual mode?