Building on prior research concerning the benefits of individual control over illuminance, this experiment tested the effects of tunable light source colour and initial light-emitting diode (LED) spectrum on office workers’ cognitive performance, mood, and satisfaction over the working day. All participants experienced a fixed spectrum in the morning (nominally 3000 K or 6500 K). Half were permitted to use a colour-tuning control to adjust the light source spectrum throughout the afternoon; the other half made one selection at the end of the day. As predicted, participants’ colour-tuning choices varied widely from one another; however, there was no influence of the starting spectrum on their choices. Participants who had colour-tuning control all afternoon used the controls more frequently than has been observed for dimming controls. Colour-tuning control led to a small increase in pleasant mood, and all participants verbally reported liking the technology. For participants with the fixed spectrum all day, there was a main effect in which 6500 K participants had faster psychomotor responses than 3000 K participants. Over the whole sample, participants did not show the expected drop in alertness and arousal during the latter parts of the day, a finding possibly attributable to the high correlated colour temperatures generally chosen with the colour-tuning system. Future research should explore the interface design for colour-tunable LEDs, provide greater experience with the system, and examine longer-term effects of dynamic changes (both automatic and individually selected) in light source spectrum.