 Color average

A uniform incoherent initial mixture of two independently prepared fundamental color charges requires an average over their $N^2$ initial possibilities, together with a sum over unobserved final color charges. This differs from preparing a coherent color singlet. For quark-antiquark annihilation through one vector in the adjoint representation, $c_s=T_{ji}^a{T}_{kl}^a$ gives $N^{-2}\sum |c_s{|}^2=(N^2-1)/(4N^2)$. For the exchange contraction $c_t=T_{ki}^a{T}_{jl}^a$, $N^{-2}\sum c_s{c}_t^{\ast }=-(N^2-1)/(4N^3)$ by the SU(N) fundamental completeness relation. The negative overlap reverses the sign of the corresponding Abelian interference term.