  Isodoublet color-octet scalar bosons appear in the low-energy limit of a
natural extension of the Standard Model in which the electroweak symmetry is
broken by a $t\bar t$ condensate. We briefly discuss the model and show that
radiative corrections (involving these scalars) to the branching ratio
$R_b=\Gamma (Z\rightarrow b\bar b)/\Gamma (Z\rightarrow {\rm hadrons})$ are
negative and thus place a stringent lower bound on the masses of the colored
scalars. This turns out to be $\sim 400$ GeV with $m_t=150$ GeV and increases
quadratically with $m_t$. It is emphasized that $R_b$ is a parameter that is
well-determined experimentally and that theoretical estimates are relatively
free from uncertainties emanating from hadronic corrections and are
comparatively insensitive to the value of the top mass.
