Constant albedo of surface is an important premise widely used for Shape from Shading(SFS). Since it is not satisfied when the surface is in multicolor, the application of SFS methods are very limited. In this paper we introduce a novel method for gray-scale transformation as a preprocess of SFS methods, which removes intensity differences between colors. First we cluster the pixels based on color using an improved Hough Transform approach, and generate Color Lines to describe color information. Then the color is adjusted using Color Lines, and the multicolor image is converted into gray-scale without sudden change of intensity between regions in different color. Experimental results show that our method outperforms existing methods such as linear luminance when 3D models are reconstructed by SFS.