The Color map from masks node uses three masks to construct the red, green, and blue channels of the output color map. Each mask value between 0.0 and 1.0 is converted to between 0 and 255.
The inverse operation is done using the Split color map node.
To add a Color map from masks node, right-click in the Graph Editor and select Create Node > Color > Color map from masks.
This node has no parameters.
Use the Fast-creation menu by clicking and dragging the output connector to find compatible nodes.