Confused about flash and gels

At a workshop a while ago, I was demonstrated a technique for balancing flash with ambient light at sunset, by a fire or candlelight, etc. It was all perfectly clear at the time, but now I cannot seem to understand the reasoning anymore, nor the details. Hopefully, someone can enlighten me

OK, so I've known about the old technique of putting a CTO over the flash to balance the flash light for the sunset. However, the technique demonstrated involved two gels: one on the flash and one on the lens. IIRC, the instructor put a green filter on the flash and a CTO on the lens. But does that make sense? And what green was it? It seems to me like the gels would cancel each other out, leaving the foreground in "daylight" and the background an exaggerated orange. No? That is definitely not the effect that was demonstrated. I don't get it...

Chris, you can use the " Mired"-system to calculate that.

It makes sense; there was the orange on the lens, making the entire shot yellowish.

Within the reach of the flash, everything was green, but with the orange on the lens, it became white.
The background - not hit by the green flash - remaind yellowish.

Exactly, within the reach of the flash, everything becomes white. This is not what I remember the purpose of the technique was. I must be remembering incorrectly, and the purpose of the technique was exactly that.

So if the orange gel was an CTO, what is the green gel called? CTG?

I think so, but I haven't used it since ages... so you better verify.
This can be done by just stacking the two filters together....

Starting point is alwith the degree of yellow (blue/red, etc) you want to have on the background = the lens filter.
Once you choosen that one, the strobe filter is easy, beeing the "counterfilter" for white.

With digital, one might draw a yellowish backgroundlayer in PS.

Thanks Michael