Swap Channels node
The Swap Channels node can be used to replace each channel (R, G, B and A) of an with 0, 1 or a channel of its input (inverted or not).

Inputs
The Swap Channels node has a single RGBA input.
Outputs
The Swap Channels node outputs an RGBA image whose channels are defined by its parameters.
Parameters
The Swap Channels node has a parameter for each channel that defines its value (0 or 1) or source (a channel of the input image, inverted or not).