With an oultine color property for multipatch layers, the edges and the faces of the objects could be rendered with two different colors. It would highlight the edges and create a better rendering on 3D scenes.
Working with ArcGIS Pro and non-textured multipatch objects, symbology properties for multipatch layers allow you to define a unique color. It is not possible to define a specific color for the outline of the faces. In many cases, the edges of the multipatch objects are not (or barely) visible on a 3D viewer. The scene ends very uniform on ArcGIS Pro or on a webscene (see attached examples).