We consider an enlarged colour sector which solves the strong CP
problem via new massless fermions. The spontaneous breaking of a
unified colour group into QCD and another confining group provides a
source of naturally large axion mass $m_a$ due to small size
instantons. This extra source of axion mass respects automatically the
alignment of the vacuum, ensuring a low-energy CP conserving vacuum.
The mechanism does not appeal to a $Z_2$ “mirror” copy of the SM, nor
does it require any fine-tuning of the axion-related couplings at the
unification scale. There is no light axion and uncharacteristically
the lighter spectrum contains instead sterile fermions. The axion
scale $f_a$ can be naturally brought down to a few TeV, with an exotic
spectrum of colored pseudoscalars lighter than this scale expected and
observable at colliders exclusively via strong interactions. The
$\{m_a, f_a\}$ parameter space which allows a solution of the strong CP
problem is thus enlarged well beyond that of invisible axion models.