Colour-preserving gluon vertex ($(T^3{)}_{rr}=1/2,(T^8{)}_{rr}=1/(2\sqrt{3})$)

A quark-gluon Feynman vertex whose initial and final quark have the same chosen color charge basis label. Such vertices exist: the diagonal Gell-Mann matrices act nontrivially on the red basis vector. Their gluons are traceless colour-octet combinations, not colour singlets. A ninth singlet generator proportional to the identity is absent from SU(3).