All of Hue Cycle’s filters are based on the Prime filter. With its default settings, it simply cycles the hue of the image coming through the camera. Uno's icon indicates that there is no gradient applied to the image. Tapping it will apply a gradient setting called Duo.
Duo applies an effect that offsets the image’s hue change in a linear fashion, from one color to another. Its direction is determined by which way you swipe over the image. In the example image, it would appear that the gradient animates upward.
Here’s a neat trick you can try: If you have the Hue Cycle app, and you are viewing this website on a separate device, use the Prime filter with the Uno setting enabled, and aim the camera towards this example image. You should be able to see the image animate as if it’s in the Duo setting, while retaining the gradient direction that was selected when this image was taken!
Triple is similar to Duo, except that the hue gradient effect fades to 3 different colors opposed to just two.
In the example image, the gradient direction animates from left to right. To change the gradient direction, swipe up, down, left or right over the image seen on screen picked up by the camera. An arrow indicating the direction will appear on screen.