That's KSLV-2 aka Nuri. The first stage uses kerolox gas generator engines so the brighter flame is almost certainly ignited fuel-rich gas generator exhaust.
This image of a KRE-075 first stage engine test shows the dark, fuel rich gas generator exhaust at the left. (image source https://www.reddit.com/r/...
Looking at the engine configurations for the Atlas V:
Credit: ULA, from Spaceflight Now article
I think it's likely a plume impingement effect from the two nozzles of the RD-180 engine. The plume expands out from each nozzle and between them has nowhere to go but out sideways, I suspect there is a nearly identical one on the other side of the rocket.