Light emitting diodes (LEDs) are an increasingly popular choice for greenhouse supplementary lighting. Many different colours of light can be produced using LEDs, but not all colours of light are used efficiently in photosynthesis. This thesis focuses on how efficiently different colours of light are used in photosynthesis – when applied alone or in combination with simulated daylight and shade – and how photosynthesis responds to short-term changes in light colour. Some attention is given to the impact of supplementary blue light on plant architecture and plant light interception as well as the impact of temperature on the efficiency of photosynthesis. A highlight of this research is that some colours of light are used much more efficiently when combined with daylight than on their own. Additionally, a biochemical mechanism in leaves is shown for the first time to positively influence the efficiency of photosynthesis when light colour changes.