Concentrate and edge color seem conflict.

I draw a simple graph as shown below and found different behavior of the color display.

digraph G {
concentrate=true
edge [dir=none]
a -> b [color=red]
b -> c
c -> b
b -> a
}
http://i.stack.imgur.com/fVKWv.png

Graph show edge between a and b is red >> this one is correct.

But when I just change it to be

digraph G {
concentrate=true
edge [dir=none]
a -> b
b -> c
c -> b
b -> a [color=red]
}

http://i.stack.imgur.com/aDQGy.png

This time the color of edge a and b is black not red color as I want. Could someone figure out what I do wrong here?

In dot, when you say

In dot, when you say concentrate=true and there are multiple edges between nodes on adjacent ranks, they are drawn as a single edge. In your case, a->b and b->a will appear as a single edge. It would appear that the color characteristics of the first edge dominate.

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