I got some news, may be useful for everybody:
looks like the probles is about “embedde color spaces”,
footage was shot with a Canon 5D Mark II, and the straight H264 output seems fine for After Effects,
but once you turn it ProRes 422 (HQ), After Effects doesn’t know how to handle it.
If I try to interpret the footage that option seems to be unavailable (embedded profile is sRGB), and my default project color space is “None”
The point is that I like and I do want colors and gamma as I see them in the AE Viewer and choosing a color space, no matter wich, messes things up dramatically.
So what I need to do is keep images just as they are, and the only solution I kinda sorted out is checking the “preserve rgb” option in the Export settings.
This thing is claimed by Adobe to “preserve the values numbers but not necessary the look”, infact in the output file the gamma is identical and the colors are a little bit warmer.
I gave up and chose this solution cause I needed to deliver, by the way the best solution I think is to set IMMEDIATELY the project settings to match the embedded color profile, or viceversa where possible, and start color ONLY THEN.
Another lesson learned cursing, but I still think there must be a way to preserve the look you got on the viewer no matter what color profiles you’re messing with.
Keep the faith