Dac*_*cav 42
digraph {
    A; B; C
    subgraph Rel1 {
        edge [dir=none, color=red]
        A -> B -> C -> A
    }
    subgraph Rel2 {
        edge [color=blue]
        B -> C
        C -> A
    }
}

您可以将dir=none边缘属性传递给无向图连接:
digraph {
    A; B; C
    
    A -> B
    B -> C
    C -> A [dir=none]
}