Ok, even if there is one point which in your eyes is exactly equidistant between green and blue, you can rate every other point accurately. Any point which is closer to green than that you pick green and any point which is closer to blue you pick blue.
Your road analogy doesn't work because the aim of this test is to choose from two binary options, not to lead you to any feasible destination.
You are not being asked "which colour do you call this?", you are being asked "is this more green or blue?"
A better road analogy would be that you are trying to direct someone westwards or eastwards and you come to a junction. It's only if the road is pointing due north or south that you can't make a binary decision. Every other road, even if it's only pointing a degree or two from a north/south axis, will take you at least slightly more westwards or eastwards.