A new version of the popular RGB model has been developed to meet the expectations of users of greenness and whiteness assessment metrics. It has been called “RGBfast” due to its simplicity and significant assessment automation. The criteria were limited to six key parameters that are easy to objectively express numerically, which combine various features of analytical method determining its functionality and sustainability. A customized Excel sheet applies the entire procedure itself after entering the appropriate input data. The assessment outcomes are presented in concise and easy-to-interpret tables, which can be used as pictograms. RGBfast enables reliable comparison of alternative procedures dedicated to the same purpose, thus can be a good support during the validation of new methods and literature reviews. As a case study, the model was applied to compare five methods for the determination of phosphate using different types of (bio)sensors. Subtle differences between them have been captured and analyzed. The mathematical structure of the model can be modified in the future, for example to take potential opportunities offered by artificial intelligence technology.