I'd love a last step in the test where you're presented with the gradient, but before showing the distribution and the user's score. Allow the user to select where they consider their threshold, then display the final results.
When I could see the entire gradient, I actually thought green continued to the right a bit more than where my line was.
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).
Does it not actually work on mobile?