The Kdenlive documentation is currently lacking any description for the Bézier curve effect. For RGB, R, G, B I suppose that I do roughly understand what is going on under the hood of this effect. However, for Hue I'm not entirely clear. Am I right that for each pixel its hue gets calculated, this is then taken as the X value of the graph. According to the curve, Y is calculated, which would be the new Hue value. Finally, Hue is converted back into RGB space? Are the X and Y axis ranges then ranging from 0 to 360 instead of 0 to 255?

Hmm ... hoped to get a good explanation from someone else. So far, I've found out that it basically seem to work as follows: the horizontal axis represents the original (input) hue (that is, "color"). It starts with red at the left end, then yellow, green, cyan, blue, pink, and finally red again.

The vertical axis states the output hue according to your curve and your input hue. Again, the vertical axis starts with red at its bottom and then changes to yellow, green, cyan, blue, pink, and red again.

As far as I understand the Hue curve effect, it can be used to translate hues. One application I've found is to slightly the hue of only some colors by punching a small bump into the otherwise straight curve. For instance, I can shift some colors more towards a red while not touching other colors and avoiding an overall tint that I would otherwise get by simply raising the red channel.