This test finds the midpoints of people's spectrum. They're not asking is "is this completely blue or completely green" but rather "is this more blue or green"
This is the wrong way to do it, psychometrically, see here: https://news.ycombinator.com/item?id=47929056. You need to provide people gradations, or you get junk responses / abandonment, and your instrument doesn't measure what you think.