One of the major issues that obstruct the consistency and interoperability of medical color LCDs is the challenge of accurately characterizing and modeling the LCD color response. The challenge originates from the instability of the primaries – the chromaticity of primaries varies with the digital driving level. The primary instability also manifests itself in other forms such as color gamut shrinkage, color shift, gray imbalance, and contrast reduction. In this paper we describe a quantitative metric, primary stability, for measuring the stability of the primaries. Two metrics were then derived from primary stability to measure the color shift of a display: (a) the area-between-curves metric measures gray imbalance, and (b) the area-under-curve metric measures the reduction in color gamut. Characterization data from 9 displays are used to demonstrate the capability of the primary stability-based metrics.