Yellow/Purple Flue gas in the stack

I was informed today by one of the on-site operators that they saw the colour of the stack flue gas change from colourless to a yellow/purple gas for a short period (~5mins) before returning back to colourless

It’s a heavy-fuel oil steam boiler (~4ton) @ 10 bar.

I know the that the colour is heavily dependent on the chemistry of the fuel being burned but has anyone come across such behaviour? I’m trying to find a place to start before jumping into a full investigation.