The issue is not the immigration officers in Colombia but the check-in staff at the airport in Brazil, notably at GRU. If the flight is overbooked or it is otherwise convenient to offload pax, check-in staff will deny you boarding on flights from Brazil to Colombia if you are unable to show proof of YF vaccination. Mind you this is just flights from Brazil to Colombia we are talking about. For flights elsewhere to Colombia, YF vaccination is not required.
I know from personal experience that it is enforced at the AV check-in counters at GRU. Luckily, I had my yellow vacc booklet with me so it was not an issue, but I witnessed others (foreigners) being refused check-in because no YF vaccine certificate/proof.