1. A method for compressing an image by classifying colors, wherein said method comprises: collecting an photo image, said photo image comprising plural rows, such as: a first row data, a second row data, and a third row data,..., and a mth row data, wherein each of said plural rows data comprises plural pixels, such as: a first pixel, a second pixel, a third pixel..., and a nth pixel, and said plural pixels are showed in a RGB coordinate; transforming said plural pixels from said RGB coordinate to become a Lab coordinate; proceeding said plural rows in turn in a row compressing process, wherein said row compressing process comprises: proceeding said plural pixels in turn in a pixel compressing process, wherein said pixel compressing process comprises: adjusting a memory if there are data in said memory or not; saving any of said plural pixels in said memory to become said data when there is no said data in said memory; calculating a first difference between said any of said plural pixels and said data when said data are in said memory, wherein said first difference is a minimum first difference when said data are an only one pixel value and said first difference is plural first differences when said data are plural pixel values; collecting a minimum first difference from said plural first differences; comparing said minimum first difference and a first set value; classifying said any pixel as same sort as a pixel which is corresponding to said minimum first difference and saving the index value of said any pixel when said minimum first difference is lower than said first set value; calculating plural second differences between said any pixel and a pixel whose location is the same row and the forward column of said any pixel, said any pixel and a pixel whose location is the same column and the forward row of said any pixel, and said any pixel and the forward row and the forward column of said any pixel when said minimum first difference is not lower than said first set value, wherein a minimum second difference is got from said plural second differences; comparing said minimum second difference and a second set value; classifying said any pixel as same sort as a pixel which is corresponding to said minimum second difference and saving the index value of said any pixel when said minimum second difference is lower than said second set value; saving said any pixel in said memory to become said data when said minimum second difference is not lower than said second set value; saving said plural pixels in said memory; and saving said plural rows in said memory.