Those are not my own samples, so I cannot really speak to that; if I were commissioned to reproduce these, I would tend slightly toward the darker end of the spectrum. Any chant books that I have where the rubrics are well, red, tend to be a vibrant red, but sometimes I take issue with this, because in the wrong lighting, that lighter shade could lack sufficient contrast. I have a folio from an Antiphonale Romanum dated 1732 in my office, and that is more of a burnt red (not maroon, but not cherry red either), although time could play a factor in that. It very well could have been more classic red 300 years ago…
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Another thing worth considering: if you use a red that is too light, and then someone photocopies it, the copy’s lines will be a washed out grey, rather than a “less dark black”.
Some publishers provide ‘photocopies’ of old scores, with a red stamp on the top, stating that it’s a licensed copy – if you try to photocopy it, the red bit doesn’t show up well (on a mono copier, at least).
Considering the relative ease of GABC these days, I think the excuse for poor quality scans of old scores is pretty non-existent. Making new editions is so easy, compared to days of yore. My copy of the triplex is inexcusable, considering the price. (Don’t get me started 
)
Well, I am not @Romanos and don’t have his expertise but think this will answer your question, which I had, too!
I agree, though I wasn’t particularly taking about plainsong – even some choral octavos from the 1950s are ‘printed’ by publishers from scans. With Dorico, the excuse for not paying a graduate to match the original is also non-existent!
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Agreed. My choir hasn’t seen an ugly score in years, because it takes me all of 30 mins tops to enter in the notes and reproduce another edition.
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Here’s a snippet from Mary Berry’s book “Cantors, a collection of Gregorian chants” where it mentions the use of coloured staff lines as well as clefs to remind singers where the semitones are. (My red box.)
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