There are no international standards to grade the strength of color change in alexandrite and the better labs may use master stones to grade the strength of a color change. Most labs use something like strong, moderate, or weak to describe the strength of a color change. For color grading, the GIA system is the most convenient and they offer a program called something like gem Ewizard that is useful. There is some variation as a function of the monitor you are using. Although the system is not perfect, it is the best practical system to grade colored gemstones and we use it.