I know they are very voltage depenedent, so it doesn't seem likely that they could be dimmed. I don't actually know, mind you, but I doubt that a dimmer can be put on the line for them.
Yes they can be dimmed by changing the voltage and they will not change color. Light will not actually run at 12 volts so as installed they will have a dropping resistor or be wired in series. Still changing the voltage will change the current flow and the brightness.
