The Mask to color map node outputs a color map corresponding to a colored mask according to the color ramp.
To add a Mask to color map node, right-click in the Graph Editor and select Create Node > Color > Mask to color map.
This node has no parameters.
The node's input connectors link to a color map and a color ramp.
Use the Fast-creation menu by clicking and dragging the input connector to find compatible nodes.
For example, the following color ramp and mask combination shown on the left outputs the terrain shown on the right.