American tally systems typically rely on the single RED light over monitors to indicate that what that monitor represents is hot or on air.  In Europe, they adopted having three tally lights:

• Red = Currently on air
• Yellow = ISO.  This warns the Technical Director that this source is currently feeding a separate record deck.
• Green = Preset by Technical Director that this source will be a part of the next take or effect.

It is quite possible to see a single monitor that has both Red and Yellow tally lights lit.  This means that the live feed (Red tally) is using this source and is also feeding an Isolated deck (Yellow tally).

I personally think the following is a possible option:

• Red = Currently on air.
• Yellow = Preset
• Green = Preview