Ok, so decided to spend my lunch break walking around a bit and getting some better mid-day shots for evolutionary comparison between the Nexus 6 (2014), Pixel XL (2016), and Pixel 2 XL (2017). Never bought the 6P, so missing that step. I tried my best to keep a steady hand and get the same shot on each, but human error will be on display with slight framing differences or motion blur.
Sets below are displayed N6 1st, Pixel XL 2nd, Pixel 2 XL 3rd. I've jotted a couple of notes in some places regarding my amateur observations and, more importantly, impressions of their respective accuracies in capturing color.
Charleston Harbor
*N6 shot was very very off here, despite a clean lens and being as still as with the other shots, so not included.
Pixel XL
Pixel 2 XL
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Rainbow Market
Noticeable difference in color of the N6/Pixel vs the Pixel 2. The Pixel 2's coloring is much closer to the true scene.
Nexus 6
Pixel XL
Pixel 2 XL
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Custom House
Nexus 6
Pixel XL
Pixel 2 XL
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Courtyard
Here the colors are truer on the Pixels again, though the Pixel XL loses a lot of detail in the heavily shadowed areas in comparison to both the Nexus 6 and Pixel 2 XL (e.g., visibility of the grating surrounding the fountain)
Nexus 6
Pixel XL
Pixel 2 XL
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St. Philip's
Nexus 6
Pixel XL
Pixel 2 XL
(not sure why, but it's shown as a 'thumbnail' below - along with that horrible N6 shot I mentioned earlier).