To alleviate the problem of the poor imaging quality of conventional color image sensors under low illumination, a low-illumination color image enhancement system based on a single sensor was proposed. The system simultaneously collects visible (VIS) and near infrared (NIR) images and enhances the VIS image using the NIR image. In the system, a series of image processing methods were proposed to handle the mixed light images. These methods involved a VIS separation method combining color subtraction and noise reduction to eliminate the influence of NIR on VIS color, a fusion method combining color space conversion and standardization processing to fuse VIS and NIR images, and a pixel replacement method to solve the contradiction between color discrimination and signal-to-noise ratio. Through the image processing of low illumination using NIR, the system can significantly improve the brightness, information, and signal-to-noise ratio of conventional color image sensors under low illumination.