A method and system for controlling electrical devices, such as lights capable of emitting different colours or colour sequences (shows). A multi-field command protocol is proposed to transmit command messages from a line controller to a controllable electrical device (light) to control its mode of operation, and optionally at least one dimension associated with the mode (e.g. light colour, and brightness and/or colour saturation, or blending colour show and speed of changing colours). The protocol comprises brief interruptions to the power supplied to the electrical device comprising a first variable length OFF time, a variable length ON time following the first OFF time, and a second variable length OFF time following the ON time. Together these times form three information fields having values represented by their respective lengths/durations. The fields define a selected mode of operation of the electrical device, and optionally at least one dimension associated with the mode.