Quantization of images containing low texture regions, such as sky, water or skin, can produce banding artifacts. As the bitdepth of each color channel is decreased, smooth image gradients are transformed into perceivable, wide, discrete bands. Commonly used quality metrics cannot reliably measure the visibility of such artifacts. In this paper we introduce a visual model for predicting the visibility of both luminance and chrominance banding artifacts in image gradients spanning between two arbitrary points in a color space. The model analyzes the error introduced by quantization in the Fourier space, and employs a purpose-built spatio-chromatic contrast sensitivity function to predict its visibility. The output of the model is a detection probability, which can be then used to compute the minimum bit-depth for which banding artifacts are just-noticeable. We demonstrate that the model can accurately predict the results of our psychophysical experiments.