Representing color images in grayscale has practical and theoretical importance. Current color-to-gray transformations seldom ensure both quality and efficiency simultaneously in practice. In this paper, we present an efficient global mapping from color to gray while preserving visually dominant features of color images. Our color-to-gray transformation is based on a variant of traditional Difference of Gaussians band-pass filter, which is called luminance filter. The band-pass filter usually has high responses on regions with discriminative colors from their surroundings for certain band. The grayscale is derived from the luminance passing a series of band-pass filters. Our method is linear in the number of pixels, simple to implement and computationally efficient, making it suitable for high resolution images. Experimental results show that our method produces convincing results for a large number of natural and synthetic images.