Qian et al. proposes a novel grayness index for finding gray pixels and demonstrate its effectiveness and efficiency in illumination estimation. The grayness index, GI in short, is derived using the Dichromatic Reflection Model and is learning-free. GI allows to estimate one or multiple illumination sources in color-biased images. GI is simple and fast, written in a few dozen lines of code, processing a 1080p image in?0.4seconds with a non-optimized Matlab code.