This paper presents a new metric for evaluating the color perceptual smoothness of color transformations. The metric estimates three dimensional smoothness to cover the full gamut of the transform. This metric predicts any artifacts like jumps in any gradient introduced by the transformation itself. From the state of the art, three works have been found and compared for evaluating their pros and cons. Based on these previous proposals, a new metric has been developed and tested with several applications. The metric is based on the perceptual distance: CIEDE2000. The defined metric is dependent on the number of ramps and the number of colors per ramp but these two parameters can be reduced to a single one called granularity. The proposed metric has been applied on the AdobeRBG and sRGB color spaces with and without the addition of artificial artifacts and tested for a large variety of granularity values. Several basic statistics have been proposed and the root mean square seems to be a good candidate for representing the global smoothness. The metric demonstrated robustness for evaluating the global smoothness of a transform and also or detecting small jumps.