We've got two varieties at work. In both cases, switching the rocker switch to the "top" position gives a white light from the conventional bulb.
The "communal" lamp we keep in the office has separate red and green buttons on top, only active when the rocker switch is in the "bottom" position. The red button "latches", for use as an emergency tail lamp, whereas the green aspect only stays illuminated whilst holding the button down.
Our personal issue lamps, meanwhile, default to red with the switch in the "bottom" position, with the green button overriding that for as long as it's depressed...Not a lot of use for Train Dispatch if you inadvertently let go of the button and end up showing the Driver/Guard a red as the train's pulling away!
As others have mentioned, battery life isn't as good as a Bardic (2.5 hours according to the instructions that came with ours), but that's only really an issue if you have to use it as an emergency tail lamp and the "proper" emergency lamp isn't available.
Cheers,
Barry