Gluon exchange

neu

This is a coursework question, but i know the answer i dont understand why it's correct.

"A proton antiproton collider produces a pair of top quarks, draw a Feymann
diagram of this process that involves a single gluon."

the answer being that 2 like quark/antiquark ( u & u bar or d & d bar)collide to produce a single gluon which decays to top quark aintiquark pair.

This seems to contradict what i've read about gluons

If gluons couple to colour charges and don't affect flavour, then how can u or d quarks (in proton) change to t quark pair by just
a single gluon exchange?