In perceptually uniform colour spaces, the perceptual differences in colour pairs are approximately the same as the Euclidean distance between them. Uniformity is of great importance in applications such as gamut mapping where the perceptual difference between original and mapped colour needs to be minimised. Ideally, in a perceptually uniform colour space, the locus of constant Just Noticeable Difference (JND) around different colour samples should be the unit sphere. While several perceptually uniform colour spaces for Standard Dynamic Range (SDR) and High Dynamic Range (HDR) have been proposed, there is not a standardized uniformity metric with respect to which we might judge whether one space is more uniform than another. In this paper, we propose and develop such a uniformity metric. Importantly, our approach takes into account changes in all three directions of a colour space including luminance and this is in contradistinction to prior art that focuses mainly on the colour signal (separate from luminance). The proposed metric can be based on any perceptual colour difference metric that models JNDs.