Code
The finish shall be at least 6.5 on the Munsell color value scale.
Annex
The State of Wisconsin uses the Munsell color chart and requires values of 6.5 or greater. The Munsell color system looks at color purity, hue, and lightness to assign a value. This system is used in other industries and information on this system is easily available.
A contractor could provide a mock-up during the submittal process to the AHJ or engineer for review and approval. Plaster and other quartz aggregate manufacturers have reflectance testing that is available for finish samples.
The American Plasterer’s Council defers to ASTM Standard E 1477 – 98a title “Standard Test Method for Luminous Reflectance Factor of Acoustical Materials by Use of Integrating Sphere Reflectometers” to determine LRV values. It's a fairly simple test method where "Test specimens are measured for (total) luminous reflectance factor by standard color-measurement techniques using a spectrophotometer, tristimulus (filter) colorimeter, or other reflectometer having a hemispherical optical measuring system, such as an integrating sphere. The specular component is included to provide the total reflectance factor condition. The instrument STANDARD is referenced to the perfect reflecting diffuser. Luminous reflectance factor is calculated as CIE tristimulus value Y for the CIE 1964 (10°) standard observer and CIE standard illuminant D 65 (daylight) or F 2 (cool white fluorescent).