Thx for the answer. <snip>
I know about your note about not using properties for everything possible, but using an old-style vertex_index property would make some things like color maps much easier. However, your way (an external property) works well also.
This is putting a property in the graph? A reason for me not to do that is that indices (as color maps) are just temporary properties; they are needed for graph algorithms to work. When vertices are added or removed, the index map should be adjusted as well. This management can be postponed if it is kept external and build the index map only just in time. So I like to put in the graph only data which conceptual belongs to the graph. The weight of an edge is probably also a candidate to put in the graph instead of keeping it external. That is the next challenge.