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Intro to Curves by Steve.
See Page 3 for the technique that Colour 2 Colour is based on.
I feel that it is rather important to understand the basic technique.
Colour From
Red, Green, and Blue points.
You can set this manually with the sliders or you can right-click in the Preview to sample a colour.
Colour To
Red, Green, and Blue points to translate to in the same manner as Steve's tutorial.
Lock To
Option to lock the Colour To sliders to a single greyscale slider.
Copy From to To
Will copy the From sliders to the To sliders.
If Lock To is turned on, they will be averaged.
Modify
While C2C is an RGB operation, I wanted some HSL way of modifying the From and To sliders. This is kind of a bizarre way of doing it, but the best I've got for the algorithm involved.
First, you have to choose which set of sliders that you want to modify. This is done via the From and To radio buttons.
Then use the HSL sliders to add or substract values for the given sliders.
Click Apply Mod and the given sliders will accommodate. The HSL sliders will also be reset back to zero.
Beer in mind that the modification range is -128 to 128 for all sliders. So, if you want to invert the hue, use H +128 or H -128. This also means that the saturation slider range is kind of funny as well.
Also beer in mind that modifying saturation is done using HSL - not Photoshop's flavor of saturation. This means that modding sat might be funny.
Right-Click
Right-clicking in the Preview window will grab RGB values and toss them into the From sliders. Seems to grab wrong values when zoomed *way* out.
Squishers
These operate much like the gamma slider in Levels, but with two notable differences:
1. They are linear as oppossed to a power function.
2. They only operate in the given range from the Colour To sliders. For example, if Colour To > Red = 100, then Squisher > Red Low will only work on the range 0 - 100 and Red High will be 100 - 255.
Squishers are something that I just wanted to toss in because I'm weird like that. Not terribly useful, but can add another layer of tweakage. The algorithm that these use isn't the greatest and I might over-haul the code one of these days.
I find Colour 2 Colour very useful for a decent range of colour cast blasting. While Steve's technique works wonders, I just don't like messing with the drop-down all the time for a simple operation.
For the most part, I right-click to sample, turn on Lock To, and tweak the last slider. That little operation works wonders in most cases with little need for the other functions.
PC/Win filter: Colour 2 Colour 
Linear Mode
