Here you are some test I've done; sources used are BD (the italian Blu-Ray - image is identical to US BD); DVD PAL (italian); ColourLike is an AviSynth script that tries to take the colors from a clip (from DVD in this case) and apply them to another clip (to BD in this case). From top to bottom:
- BD
- BD retimed by KingKong650
- DVD PAL
- BD ColourLike
- BD luma + DVD chroma
Here ColourLike script did a good job; as you can see, as in this scene the DVD is more cropped, in the last image (BDluma+DVDchroma) the colors "bleed"
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Here, BDluma+DVDchroma is better than the Colurlike script.
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Here ColourLike did a bad job, while BDluma+DVDchroma is good.
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Using one of these two methods (ColourLike or BDluma+DVDchroma) it's possible to retain the BD resolution with DVD color timing; now, if the DVD color timing is right, this methods could be used to do a proper restoration.