Good rule for contrast: If you would refer to the text color and background color by the same word, that's not enough contrast. Compare the good contrast examples, largely black on white or white on black, with the bad examples, universally gray on gray (or off-white on off-white).
You might be right! I am colorblind, and for example I can't see the red component of violet, so most "violet" is just "blue" to me. Purple, too: I have trouble distinguishing the blue in purple, so a lot of purple simply looks "red". If it were up to me, there wouldn't be a word for violet, because it doesn't exist.
This rings a bell inside my head; given people from a language/culture with two different words for what English speakers would identify as barely different shades of the same colour, they are objectively better at identifying those two shades than the native English speaker. Which essentially gives the answer "yes" to your question.
Particularly relevant quote: "This is the first time that evidence has been offered to show cross-linguistic differences in colour perception in an objective task"
This "Russian experiment" is a bit weird, as though it's true that "синий" usually refers to darker colors and "голубой" to the lighter ones, both colors to chose from could be referred to as "синий" or "голубой" interchangeably. Actually, it's not that clear what is "синий" and "голубой" anyway: sky is like The "синий", but quite often referred to as "голубой" instead, but somebody's eyes almost never are referred to as "синий": maybe only if you want to emphasize how unusually bright they are. But if your dress would be the same color as the eyes, it could be called "синий" easily.
What I'm trying to say, there's no that big difference between these two words in Russian to explain the experiment results, unlike what was shown to us on that video about some African tribe. More like "purple" and "violet" — there clearly is a difference, but many people would fail even to point which one is which, and some might even argue about that.
