I think an HSV color wheel (usually from blue to violet) with black instead of violet would be nice...
HSV refers to hue, saturation, value, but we're only really concerned with the outside edge of the wheel, where only hue is changing
Finally, we imagine using different gradients for different effects and analysis, so abstracting the wheel itself as an image would be cool (where the x-dimension of the image is the % value) so we could offer different gradients as options.
However, for the purposes of the quick demo, I'd just use one gradient and keep things simple -- one demo in hand is worth two in the bush, or something.
Jeff