Red beam lasers have been around for some years now, but more recently green laser level beams have become available.
Green Beam Lasers are up to 4x brighter than red when using them as a visible laser dot or line indoors. Outdoors in full sun you cannot realistically see any construction laser dot or line (red or green) more than a hand full of meters and that is where your electronic laser receiver comes in, to see the laser for you. So for Outdoor work Red or Green makes little difference (slight extra range with green).
So going back to the opening statement “when to go green?” the answer is when you want to use your Laser indoors visibly without the receiver. What about cost? To produce a green laser beam a green laser diode is used, these diodes are currently substantially more expensive than the more common red variety. For rotating lasers typically only one diode is used so the additional cost for green is relatively small. In multiline lasers there can sometimes be in excess of 6 diodes so to make an all green beam multi line laser can be double the overall cost compared to the red beam version.
One other factor to keep in mind when selecting, particularly, a rotating laser level is if you plan to buy a machine mounted receiver either now or at some point in the future. Most machine mounted receivers at present do not work with green rotating beams, to be precise they sort of work but at a much reduced range. So in this case it is best to buy a red beam as apposed to a green laser level.