The first (and easiest) thing I wanted to do was a paint program test. Taking jerryshadoe's cue, I went with Miracle On 34th Street. From the available DVD and Blu-ray of that movie, the final result should be spectacular.
For me, it was off to YouTube to look for standard-def colorized and high-def B&W sources. Pickings' were pretty slim but I found the needed matching snapshots. The SD 360p colorize was pretty bad -- from weak, wrong-tinted, and color-smeared video tape, with massive YouTube compression. The HD 720p was better but it had the poster's logo-bug on it (I mean, really?).
Regardless of the sources, you should always fix each with a critical eye to make it as pristine as it can become. Then the smaller colorized source would be resized and repositioned to match the B&W source. Black-border the clean picture area of each to prevent weird coloring in garbage-filled edges. And, for the best end result, correct the luminances of one or both sources to match your ideal picture, before splitting/recombining the H-S-L parts.
What to expect? Something like this (shown here half-sized to fit this page):
YouTube SD @ 360p (needs fix-ups) YouTube HD @ 720p
for Hue and Saturation for Luminance
HS & L recombination for SD colorization (fixed) at HD resolution
Of course, the caveats for this method:
* sources with different aspect ratios requiring image crop or fill-in of missing color (note outlines)
* non-uniform sizes from the masters requiring piecemeal resize-matching