This can be a very tricky node to get to know. Even the Nuke handbook documentation on this is sketchy and incomplete (IMHO).
We can conclude from this simple demonstration that the perception of the amount of hue within a color (its chroma) is linked to two things: saturation and lightness. When adjusting color it is important to have separate control over these values, which is something that the HueCorrect node offers. This effect it masks by two further values: hue and saturation. Its interface offers control over nine values:
This can change the saturation value of an image, with respect to particular regions of hue.
This can change the luminosity (i.e. brightness) value of an image, with respect to particular regions of hue.
Luminance components (red, blue, green)
This can change the r, g and b channels of an image, with respect to particular regions of hue.
Suppression (r_sup, g_sup, b_sup)
This is similar to adjusting the luminance components, but instead of nullifying them (replacing them with black), replaces them with white.
Saturation threshold (sat_thrsh)
This only effects the image if first the 'Luminance' or 'Luminance components' have been adjusted. Adjustments to this value will act as 'per hue' saturation level mask to the effect.
The curves rainbow gradient interface (above) has a single yellow vertical line running through it that represents the average of any hue value selected in the Viewer. It works in much the same way as the R, G and B lines that you can see in the ColorLookup window and is best used in conjunction with a square selection within viewer window (shift select). Running at right angles to the hue line, there is a grey horizontal line that represents the saturation level of the selected region.
The curves in the window can not only be changed one by one, but also as a group (i.e. all selected curves moved 'ganged up' in an identical fashion) and in opposition (e.g. green sat an lum moved up whilst g_sup is moved down). Generally the sat_thrsh should be moved last as it is a mask value which effects the generalities of the node’s effect.
In compositing this node is used to ‘spill suppress’ green screen footage (i.e. remove unwanted green light reflected from the green screen). There are many ways in which it can be used but I recommend that you change its values sequentially and re-set them frequently (i.e. start again) till you have understood what you are doing. I also highly recommend first testing this out on a HueWheel rather than an image.