A method of controlling a lamp comprising first, second and third colour LEDs and a white LED, the method comprising: characterising the variation of chromaticity and luminosity of each of the LEDs as a function of temperature over an operating temperature range; characterised by further comprising defining each of a virtual first, virtual second and virtual third LED, such that the chromaticity of each virtual LED can be achieved by combining component light from the first, second and third LEDs for all temperatures within the operating range; defining a virtual white LED, such that the chromaticity of the virtual white LED can be achieved by combining light from the white LED with light from a two of the first, second and third LEDs, for all temperatures within the operating range; receiving data representative of a requested setting R, G, B of each of three primary colours, thereby defining a requested chromaticity and a requested luminance; determining an operating temperature of each LED; determining a virtual white control setting corresponding to a maximum fraction of a total luminance at the requested chromaticity which can be provided by the virtual white LED; determining a control setting for each of the respective first, second and third virtual LEDs, in dependence on the difference between the requested setting of the respective primary colour and the control setting of the virtual white LED; controlling each of the first, second and third LED with a respective output control setting which is a sum of the respective first LED, second LED or third LED component light of the virtual white, virtual first, virtual second and virtual third LED control settings at the operating temperature; and controlling the white LED with an output control setting which is the white LED component of the virtual white LED. characterising the variation of chromaticity and luminosity of each of the LEDs as a function of temperature over an operating temperature range; characterised by further comprising defining each of a virtual first, virtual second and virtual third LED, such that the chromaticity of each virtual LED can be achieved by combining component light from the first, second and third LEDs for all temperatures within the operating range; defining a virtual white LED, such that the chromaticity of the virtual white LED can be achieved by combining light from the white LED with light from a two of the first, second and third LEDs, for all temperatures within the operating range; receiving data representative of a requested setting R, G, B of each of three primary colours, thereby defining a requested chromaticity and a requested luminance; determining an operating temperature of each LED; determining a virtual white control setting corresponding to a maximum fraction of a total luminance at the requested chromaticity which can be provided by the virtual white LED; determining a control setting for each of the respective first, second and third virtual LEDs, in dependence on the difference between the requested setting of the respective primary colour and the control setting of the virtual white LED; controlling each of the first, second and third LED with a respective output control setting which is a sum of the respective first LED, second LED or third LED component light of the virtual white, virtual first, virtual second and virtual third LED control settings at the operating temperature; and controlling the white LED with an output control setting which is the white LED component of the virtual white LED.