A polychromatic correlation detection technique by complex spatial filtering is presented. This technique utilizes a diffraction grating and three co-lineared (red, green and blue) coherent sources. The techniuue of complex spatial filter synthesis for the polychromatic correlation detection is demonstrated. This technique offers true color correlation detection which is very suitable for color signal recognition and identification. The correlation diffraction efficiency with this technique is generally higher than that of the wavelength-multiplexed technique. Several interesting experimental demonstrations if this color signal correlation detection scheme are provided. Finally we note that this color signal detection is a simple and versatile processing technique which has broad range of applications of complex color signal detection, recognition and identification.