A method for measuring a color property of a pressurized flowing liquid is characterized by contacting the liquid under test against a transparent partition spaced a predetermined distance from a transparent window. The partition has an index of refraction and a thickness dimension that is less than that of the window. At least some of the radiation reflected from the liquid undergoes total internal reflection within the partition while, simultaneously, evanescent coupling of that reflected radiation into the material of the window is prevented. Evanescent coupling into the material of the window is prevented by: i) disposing a medium having an index of refraction less than that of the partition between the window and the partition, and ii) maintaining the spacing between the window and the partition to a distance not less than three (3) times the wavelength of the interrogating radiation.