Seems like something that colormatrix would fix. But first, just to make sure, are you directly editing with clips that are encoded with a lossy codec? A common example would be .avi files with a divx or xvid codec. If that's the case, then I suggest you to take a read of this
to understand why you shouldn't and also to Read ErMaC & AbsoluteDestiny's Friendly AMV Guides Lovingly Overhauled Largely by Zarxrax
to learn how to do things properly.
If I am wrong though, and you are indeed editing with lossless clips or through fake avis or whatever, then I suggest you to give colormatrix()
a try, as it seems to be an issue with the color coefficients detection.