Use Combine to combine the individual color space channels of three different source clips (RGB, YUV, or HSL).
To access the Combine menu, use:
Batch, then select a node from the Node bin.
Timeline, then use Batch FX (see Creating Batch FX).
Tools, then select from the menu (See Accessing Tools).
The Modular Keyer, then select a node from the Node bin.
This node accepts a front, back and matte clip, and outputs a result.