if your looking to paint on Import assets like what type of paint (Vertex paint , paint colors ,paint textures , paint foliage onto assets, paint landscape layers ) you need to clarify . if you are talking about texturing the asset and painting the mesh just do it that in Substance or any alternative and then import after .
if talking about painting layers on the landscape you use the layer blend Node in the materuial graph to create a multi layered paintable Landscape Material
i will try help you if you can clarify.