_,,,^..^,,,_ said:

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.

The problem with both these techniques is that the sources are so different in both luminance and color, their selective recombination still suffers from the Blu-ray's foibles. Getting luminance is not the same thing as getting resolution.

Fortunately, there is an alternative -- color correction. As long as this was not some George Lucas re-imagined "director's cut" hi-def, a single correction might be usable over the entire movie. Worse, some scene-by-scene exceptions. Anyway, here's a very quick proof-of-concept going by the numbers (of the histogram graphs):

This is what needs to be changed -- everything except the resolution. :)

This DVD is the target -- an end result, only in higher resolution. Notice the RGB graphs. These are what is to be duplicated on the Blu-ray. By eye, it is easy to use histogram functions to squeeze down and slide around each Blu-ray graph to roughly match it's DVD counterpart:

HISTOGRAM
RED       Low 16 Gamma 1.2 Midtones +25
GREEN   Low 16 Gamma 1.2 Midtones +25
BLUE      Low 12 Gamma 1.3 Midtones +20

The original Blu-ray is in grey and the red overlay is the result of spectrum squeeze-down ("Midtone: compress") and slide-around ("Low" and "Gamma"). Done (except for fine tuning the numbers -- always use an eye-dropper for dispassionate precision)!

Of course, if you wanted to repair any Blu-ray crush or blow-out, those areas would be patched in from the non-crushed, non-blown-out DVD itself.