1: Add a Bitmap Output and save your Coastal Overlay color map.
2: Import that Color Map into UE4 and create a new Material and add that Texture into that Material and connect that Texture to BaseColor.
3: Inside that Material add a LandscapeCoords node and set its Mapping Scale to the size of your Landscape.
4: Finally connect the LandscapeCoords node to the UVs of that Texture Sample in your material and Save the material and apply it to your Landscape. 
Thats it! Basic color map on your Landscape. But to create a good looking Landscape requires a lot of work. I highly recommend this tutorial.