A paper presented by the NPIRI Color Measurement Task Force at the 1993 annual TAGA meeting reports that colorimetric measurements made with spectrophotometers manufactured by the same company and with the same geometry can differ by 0.7 to 1.7 CLab ΔE and that values from instruments made by different manufacturers with the same geometry can differ by 1.5 to 3.0 ΔE. The author of this poster has published extensively on the physical mechanisms that can cause such measurement differences. This poster paper illustrates the nature of these mechanisms along with other error producing phenomena such as wavelength error, thermal and light exposure color effects, fluorescence, and transient color effects produced by flash sources.