With the need of a tool for efficient nitrogen (N) fertilizer management in the rice field, this paper proposes a low-cost compact single-wavelength based colorimeter that can be used to indicate the specified six color levels of a rice leaf associated with the desired amount of N fertilizer for the rice field. Our key design is in a reflective optical architecture that allows us to investigate the amount of light scattered from only one side of the rice leaf. We also show how we implement this needed rice leaf color analyzer by integrating an off-the-shelf 562-nm wavelength light emitting diode (LED), a silicon photodiode, an 8-bit microcontroller, and a 6×1 LED panel in a compact plastic package. Field test results in rice fields confirm that leaf color levels of 1, 2, 3, 5, and 6 are effectively identified and their corresponding amount of N fertilizer can be determined. For the leaf color level of 4, our single-wavelength based rice leaf color analyzer sometimes indicates a higher color level of 5 whose suggested amount of N fertilizer is equal to that for the leaf color level of 4. Other key features include ease of use and upgradability for different color levels.