Whole slide imaging (WSI) has various uses, including the development of decision support systems, image analysis, education, conferences, and remote diagnostics. It is also used to develop artificial intelligence using machine learning methods. In the clinical setting, however, many issues have hindered the implementation of WSI. These issues are becoming more important as WSI is gaining wider use in clinical practice, particularly with the implementation of artificial intelligence in pathological diagnosis. One of the most important issues is the standardization of color for WSI, which is an important component of digital pathology. In this paper, we review the major factors of color variation and how to evaluate and modify color variation to establish color standardization. There are five major reasons for color variation, which include specimen thickness, staining, scanner, viewer, and display. Recognizing that the color is not standardized is the first step towards standardization, and it is difficult to ascertain whether the appropriate color of the WSI is displayed at the reviewers' end.