A supplemental device comprising: a connector configured to attach the supplemental device to an injection device in use; characterised in that the supplemental device further comprises: a plurality of light sources each being configured to generate light of at least one wavelength that the other respective light sources cannot, which in use can be directed out of the supplemental device onto a surface portion of the injection device for respective exposure times, the surface portion having a particular colour; at least one sensor for generating sensor outputs indicative of the respective intensities of light of different wavelengths reflected from the surface portion in use during the respective exposure times; and a processor configured to: control the plurality of light sources to illuminate the surface portion with light of first to third different wavelengths for first to third respective exposure times; obtain first to third values for sensor outputs corresponding to the intensity of light of the first to third different wavelengths incident on the sensor during the first to third respective exposure times; perform one or more calculations using at least the first and second values to provide one or more further values; and use the third value and the or each further value, but not the first and second values, to determine a property of the injection device associated with the colour of the surface portion. a connector configured to attach the supplemental device to an injection device in use; characterised in that the supplemental device further comprises: a plurality of light sources each being configured to generate light of at least one wavelength that the other respective light sources cannot, which in use can be directed out of the supplemental device onto a surface portion of the injection device for respective exposure times, the surface portion having a particular colour; at least one sensor for generating sensor outputs indicative of the respective intensities of light of different wavelengths reflected from the surface portion in use during the respective exposure times; and a processor configured to: control the plurality of light sources to illuminate the surface portion with light of first to third different wavelengths for first to third respective exposure times; obtain first to third values for sensor outputs corresponding to the intensity of light of the first to third different wavelengths incident on the sensor during the first to third respective exposure times; perform one or more calculations using at least the first and second values to provide one or more further values; and use the third value and the or each further value, but not the first and second values, to determine a property of the injection device associated with the colour of the surface portion. a plurality of light sources each being configured to generate light of at least one wavelength that the other respective light sources cannot, which in use can be directed out of the supplemental device onto a surface portion of the injection device for respective exposure times, the surface portion having a particular colour; at least one sensor for generating sensor outputs indicative of the respective intensities of light of different wavelengths reflected from the surface portion in use during the respective exposure times; and a processor configured to: control the plurality of light sources to illuminate the surface portion with light of first to third different wavelengths for first to third respective exposure times; obtain first to third values for sensor outputs corresponding to the intensity of light of the first to third different wavelengths incident on the sensor during the first to third respective exposure times; perform one or more calculations using at least the first and second values to provide one or more further values; and use the third value and the or each further value, but not the first and second values, to determine a property of the injection device associated with the colour of the surface portion. control the plurality of light sources to illuminate the surface portion with light of first to third different wavelengths for first to third respective exposure times; obtain first to third values for sensor outputs corresponding to the intensity of light of the first to third different wavelengths incident on the sensor during the first to third respective exposure times; perform one or more calculations using at least the first and second values to provide one or more further values; and use the third value and the or each further value, but not the first and second values, to determine a property of the injection device associated with the colour of the surface portion.