GHP[p, a, b] gives the ghost propagator where
a and b are the color indices.
GHP[p] omits the \delta
_{ab}.
Overview, GhostPropagator, GluonPropagator, GluonGhostVertex.
GHP[p, a, b]\Pi _{ab}(p)
GHP[p] // Explicit\frac{i}{p^2}
GHP[p, c1, c2]\Pi _{\text{c1}\;\text{c2}}(p)
StandardForm[FCE[GHP[-k, c3, c4] // Explicit]]
(*I FAD[k] SD[c3, c4]*)