The presented notes aim toward improved models of the scalar visual response to flat-field stimuli, and arc prompted by unease over the complexity of existing colour difference models. Some of the basic assumptions of colorimetry are examined in detail, and analytical methods whereby these assumptions can be investigated experimentally are presented. A key finding is that the standard CIE colorimetric model is verifiably correct as a predictor of point colour identity and metameric visual equivalence. It is however demonstrated to be a misleading basis for modeling colour difference, and an alternative scalar reference basis and experimental analysis method are presented (c) 2009 Wiley Periodicals, Inc. Col Res Appl, 34, 163-169, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20479