Color differences are determined by illumination, the spectral reflectance of objects, and the spectral sensitivity of the imaging sensor. We explore the optimal illumination conditions that best separate one object from another. Given two objects with distinct spectra, we derive the optimal illumination spectrum to maximize their color distance with a plain RGB camera. In practice, it is crucial to compose the most appropriate illuminations using available lighting sources, since creating an arbitrary illumination spectrum is unrealistic. Therefore, we derive the optimal linear combination of the provided illumination sources. Finally, we verify the effectiveness of the methods through experiments.