Feynman-gauge adjoint propagator ($D_{\mu \nu }^{ab}(p)=\frac{-i{\eta }_{\mu \nu }{\delta }^{ab}}{p^2-iϵ}$)

With a mostly-plus metric signature, $e^{iS}$, and gauge-fixing term $-(\partial \cdot A{)}^2/2$, the free adjoint gauge-field kernel is $-p^2{\eta }_{\mu \nu }{\delta }_{ab}$. Inverting it gives the displayed Feynman propagator. This convention uses the same denominator and sign as a massless real scalar propagator.