I came across an article in the Daily Mail about a phone app that can be used to measure skin colour. It is produced by Fujitsu. Trying to measure colour using a digital camera is difficult. The RGB values you obtain depend upon the lighting and the settings of the camera and even on the make of the camera. Technically, we say that the RGB values are device-dependent. Fujitsu have got around this by using a mask (see picture) that contains some standard colours that are skin tones. Presumably the app grabs the RGB values of the standard colours and then uses these to make an adjustment to the captured RGB values of the actual skin. It’s very clever and I am impressed.