  Given a set $\mathcal{H}$ of graphs, let $f_\mathcal{H}^\star\colon
\mathbb{N}_{>0}\to \mathbb{N}_{>0}$ be the optimal $\chi$-binding function of
the class of $\mathcal{H}$-free graphs, that is,
$$f_\mathcal{H}^\star(\omega)=\max\{\chi(G): G\text{ is }
\mathcal{H}\text{-free, } \omega(G)=\omega\}.$$ In this paper, we combine the
two decomposition methods by homogeneous sets and clique-separators in order to
determine optimal $\chi$-binding functions for subclasses of $P_5$-free graphs
and of $(C_5,C_7,\ldots)$-free graphs. In particular, we prove the following
for each $\omega\geq 1$:
  (i) $\ f_{\{P_5,banner\}}^\star(\omega)=f_{3K_1}^\star(\omega)\in
\Theta(\omega^2/\log(\omega)),$
  (ii) $\
f_{\{P_5,co-banner\}}^\star(\omega)=f^\star_{\{2K_2\}}(\omega)\in\mathcal{O}(\omega^2),$
  (iii) $\
f_{\{C_5,C_7,\ldots,banner\}}^\star(\omega)=f^\star_{\{C_5,3K_1\}}(\omega)\notin
\mathcal{O}(\omega),$ and
  (iv) $\ f_{\{P_5,C_4\}}^\star(\omega)=\lceil(5\omega-1)/4\rceil.$
  We also characterise, for each of our considered graph classes, all graphs
$G$ with $\chi(G)>\chi(G-u)$ for each $u\in V(G)$. From these structural
results, we can prove Reed's conjecture -- relating chromatic number, clique
number, and maximum degree of a graph -- for $(P_5,banner)$-free graphs.
