In theory you have to ignite the target with a specific weapon and set that weapon to have green flames via attributes.
So you have to create a flamethrower, put it anywhere in the world doesn't matter, don't even have to give anyone owner nor equip it, and then call "CTFPlayerShared::Burn" with that weapon. SourceMod provides a "TF2_IgnitePlayer" function which does exactly that but it does not support passing the weapon entity (Yet, maybe someone will make a PR for it, see
SourceMod Github Repo). So for now you would have to SDKCall it manually probably.
Note: For the duration parameter you would have to be careful because it scales with the weapon entity based on increased or decreased burn time