Although you are definitely right, I’m pretty sure @lof102 knows about these differences. I recently made my Mew variations list here (and @lof102 even knew some Mews I missed in my list), and he also has close contact with Sarah (@skinst ).
@lof102 first had some back and forth PMs with me yesterday before he made this post. I think these color differences can be found in any set, although the Base Set and XY Evolutions are the most notorious for color differences.
When you take a look at the 59 unique Pikachu cards picture I made about halve a year ago:
Click for a bigger image.
(got almost 80 variants now 
), you see a lot of color differences. (Ignore the sun-bleached and Albino-border ones.) But even without those you can clearly see the very first card has darker borders than some others. You can also see the difference at the 7th and 8th card of the second row (the two 1st edition Spanish Pikachus - both ©1999 and ©1999-2000).
With XY there are also color differences, where the Pikachu have three major differences (regular purple, blue, and maroon):
(Picture from @fritz 's collection.)
(I’m still looking for the Maroon one.) Although there are also different shades of the purple one, since I have a darker and lighter one:
Greetz,
Quuador