In order to improve discriminative power and illumination invariance, local color descriptors have been proposed. However, the high dimensionality of color descriptor limits the performance of matching in terms of speed. It is desirable to reduce the dimensionality for efficient computation and compact representation. A compaction of color descriptor is presented. The color feature vector is represented as a quaternion vector and then projected into a precomputed eigenspace. The resulting descriptor is highly compact and the experimental results show that the performance is almost identical to that of the standard RGB-SIFT.