An evaluation for objectively assessing the target detectability in night vision color fusion images is proposed. On the assumption that target detectability could be modeled as the perceptual color variation between the target and its optimal sensitive background region, we propose an objective target detectability metric in CIELAB color space defined by four color information features: target luminance, region perceptual luminance variation in human vision system, region hue difference, and region chroma difference. Experimental results show that this proposed metric is perceptually meaningful because it corresponds well with subjective evaluation.