Graphviz Dot, mix directed and undirected

digraph {

    A; B; C

    subgraph Rel1 {
        edge [dir=none, color=red]
        A -> B -> C -> A
    }

    subgraph Rel2 {
        edge [color=blue]

        B -> C
        C -> A
    }

}

You can pass dir=none as an edge property to the undirected graph connections:

digraph {

    A; B; C
    
    A -> B
    B -> C
    C -> A [dir=none]
}