That is the point of the exercise though. Is 50% really where you draw the line?
For some people's language usage, blue and green are adjacent colors, and thus defining a point that divides them is perfectly fine.
For other people, these are not adjacent -- for some people, there's a single color (aqua? turquoise?) between them, and green and turquoise are adjacent colors, as are turquoise and blue, and it's reasonable to ask about a dividing point between those adjacent pairs.
For those who don't use language this way -- do you consider red and blue adjacent, or do you consider purple (violet?) a necessary intermediate? Are you comfortable defining a point between red and blue, or are you instead comfortable defining a point between red and purple, and a point between purple and blue?
And for all I know, there are people for whom blue and green (or blue and red) have a distance greater than one, or greater than two...
Russian speakers broadly consider sky blue / turquoise / cyan a distinct color right between green and blue
https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction...
Although the question "is the color distinct and basic or just a shade?" is very subjective. Is pink distinct or a shade of red/purple? Is purple distinct or a shade of red/blue? Is green distinct or a shade of blue? (it's well-known that in Japanese green separated from blue only relatively recently, with very bluish traffic lights and other quirks included)
I think there's another set of questions here -- why is "blue-green family" a thing in your mind, rather than "blue-yellow family"? Is there a "red-blue family"? "Orange-blue"?
Also, yellow-blue and red-green are opponents that can't be mixed because of how our retinas preprocess the signals from cones. Therefore, you obviously end up with blue-green (cyan), red-yellow (orange), yellow-green (lime) and blue-red (magenta, which actually doesn't exist on the light spectrum) families of related colors.
The analogous version in black and white is "is this dark grey or light grey?" because that's the one asking you to guess which side of the 50/50 split the color is on.