To access the Colour Corrector, you must load clips of the same resolution. If the clips you want to load have different resolutions, resize them so that they have the same resolution.
You can load a front clip, a front and back clip, or a front, back, and matte clip for colour correction. Changes in colour are applied to the front clip.
To access the Colour Corrector menu, use:
Batch, then select a node from the Node bin.
Timeline, then use Batch FX (see Creating Batch FX).
Timeline, then add Timeline FX from the Effects ribbon (see Adding Timeline FX).
Tools, then select from the menu (See Accessing Tools).
Modular Keyer, Action or Paint tool.