Figure 4.

The colour scheme to identify the graphs in ${\mathcal{H}}_3$. (a) The shades of cyan, magenta and yellow identified with each directed edge of K₃. If multiple edges are present, the colours are added. Examples of graphs $\mathbf{H}\in {\mathcal{H}}_3$ in their associated colours, as well as the corresponding graph numbers ν(H), as defined in (5.1), are shown in (b). The subgraphs where all edges to/from a given node are present (and no others) are associated with the colours red, green and blue. The symmetry that permutes the three nodes acts on the colour scheme by permuting the colour channels. Hence, graphs which are invariant under this symmetry operation have a colour that is invariant under permutation of the colour channels; this includes white for ∅₃, black for K₃, and shades of grey for the directed cycles C_1,2,3, C_3,2,1. (Online version in colour.)