 Quark-gluon vertex

Expanding $\bar{q}i{\gamma }^{\mu }{D}_{\mu }q$ gives a vertex joining a quark, its conjugate line and a gluon. In the convention ${\mathcal{L}}_{\mathrm{int}}=-g_s\bar{q}{\gamma }^{\mu }{T}^{a}q{A}_{\mu }^a$, its rule is the displayed matrix. The gluon changes the color charge state through the generator $T^a$ but preserves the quark flavour. With distinct flavours in $q\bar{q}\to q^{\prime }{\bar{q}}^{\prime }$, this permits the tree-level annihilation graph but not a flavour-changing exchange along one quark line.