  Features of the dressed-quark-gluon vertex and their role in the gap and
Bethe-Salpeter equations are explored. It is argued that quenched lattice data
indicate the existence of net attraction in the colour-octet projection of the
quark-antiquark scattering kernel. This attraction affects the uniformity with
which solutions of truncated equations converge pointwise to solutions of the
complete gap and vertex equations. For current-quark masses less than the scale
set by dynamical chiral symmetry breaking, the dependence of the
dressed-quark-gluon vertex on the current-quark mass is weak. The study employs
a vertex model whose diagrammatic content is explicitly enumerable. That
enables the systematic construction of a vertex-consistent Bethe-Salpeter
kernel and thereby an exploration of the consequences for the strong
interaction spectrum of attraction in the colour-octet channel. With rising
current-quark mass the rainbow-ladder truncation is shown to provide an
increasingly accurate estimate of a bound state's mass. Moreover, the
calculated splitting between vector and pseudoscalar meson masses vanishes as
the current-quark mass increases, which argues for the mass of the pseudoscalar
partner of the \Upsilon(1S) to be above 9.4 GeV. The absence of
colour-antitriplet diquarks from the strong interaction spectrum is contingent
upon the net amount of attraction in the octet projected quark-antiquark
scattering kernel. There is a window within which diquarks appear. The amount
of attraction suggested by lattice results is outside this domain.
