In your example:
get(boost::edge_index, graph)), //CHANGED
They call the following constructor from here:
Which means, that what you think are classes are indeed the colors.
According to the documentation,
If the color of a vertex after running the algorithm is black the
vertex belongs to the source tree else it belongs to the sink-tree (used
for minimum cuts).
Now if you look here at the default color map, the colors are an enum
going from 0 to 4, where 4 is black.
With all of this, you can conclude that 4 is indeed the source,
but everything else (including the 1's) belong to the sink!
The different color probably depends on the implementation itself, but I
don't think you can make any assumptions about the 1's without knowing what
the implementation is...