You_Too, thank you for posting the Avisynth version, but I take issue with this part:

And to those really into color correcting: The last line should be "ConvertToYV12()" and not have "(matrix="rec709")" added. I tried this first but the colors were messed up. It was enough to have it at the first color conversion line.

This is incorrect. When you convert back to YUV from RGB, you have to use Rec.709 coefficients if you intend to output HD. ConverttoYV12() assumes Rec.601 coefficients and would only be correct for DVD. If the colors were messed up in the script, then something else went wrong, but it is not enough to specify the matrix only upon the initial conversion to RGB. You have to take this account every time you convert between RGB and YUV.