A colour display system including a cathode ray display tube (10) of the kind which has an electron multiplier (16) arranged adjacent a luminescent screen (14) comprising a repeating pattern of three different colour phosphor elements and pairs of colour selection electrodes (38, 40) associated with each channel of the multiplier operable to control the direction of the electron beam from the channels for colour selection purposes uses a switching bridge circuit comprising semiconductor switching devices (52, 54, 56, 58), e.g. MOSFETs, connected between voltage supplies for switching the voltage applied to each one of the pair of electrodes between three respective levels in synchronism to achieve a predetermined colour selection sequence. The semiconductor devices are controlled by synchronised waveforms from a pulse generating circuit (86) via optocouplers (88) enabling the switching bridge circuit to be floated at a high potential. The switching voltages applied to one of the selection electrodes may be offset with respect to those applied to the other electrode.