If a Material has a texture assigned as either the diffuse or normal map in the 3D application, those textures will be imported into Unreal as long as the Import Textures setting is enabled in FBX Import Properties.
A Texture Sample expression will be built in the newly-created material in Unreal Editor and the imported texture will be assigned to that Texture Sample. A Texture Coordinate expression will also be added to the material and connected to the UVs input of the Texture Sample. However, you will need to connect some textures to their material slots.
If the textures applied to the material in the 3D application are in a format that is not compatible with Unreal, or are connected to an unknown material attribute (for example, Diffuse in Maya), they will not be imported. In this case, and also if no textures were present in the material to begin with, the material in Unreal Editor will be populated with a randomly-colored Vector Parameter.