While the magnitude of colour gradients is well defined, its direction is not. The ambiguity in the direction is due to our inability to decide if red colour is greater than green in the same way we can say that white is brighter than grey. In this paper, we present a method to calculate the maximum signed gradient and integrate it to a grey image. Experiments on natural photos show that seventy per cent of edges are due to luminance change, i.e., all the channels change in the same direction. The other gradients are due to material change such as yellow to blue or green to orange. The algorithm can be used to decompose images into luminance and chrominance. It produces noticeably better local contrast than normal luminance conversion.