[pminimax]

That's fun! I bet people would tend to nudge the threshold toward the middle of the scale. Or you could do a sorting interface, etc.

[ddejohn]

A sorting interface would be another neat step! And yeah, I think most would gravitate toward the middle. Seeing how "far off" you are would be fun :)

Ooh maybe have the user slide a gradient left and right inside a window, aligning the center of the window with where they think the line is between blue and green (i.e., instruct the user to fill the window with equal amounts of green and blue).

[martyvis]

This test gets you sort hues along a gradient. https://www.xrite.com/hue-test

[Veve]

Ilovehue and ilovehue 2 are excellent mobile games around this sorting idea, they're quite zen and for all ages, highly recommendend!