Optical microscopy (OM) methods have been commonly used as a convenient means for locating and identifying few-layer graphene (FLG) on SiO 2/Si substrates. However, it is less clear how reliably optical images of FLG could be used to determine the sample thickness. In this work, various OM methods based on color differences and color contrasts are presented and their reliabilities are evaluated. Our analysis shows that these color-based OM methods depend sensitively on certain parameters of the measuring system, particularly the light source and the reference substrate. These parameters have usually been overlooked and less controlled in routine experiments. From evaluating the performance of these OM methods with both virtual and real FLG samples, we propose some practical guidelines for minimizing the impact of these less-controlled experimental parameters and provide a user-friendly MATLAB script for facilitating the implementation.