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In-season canopy estimation and crop stress detection are important for site-specific crop management. Before canopy closure, canopy spectral reflectance measurements taken by a spectrometer from nadir position is actually a mixture of crop canopy and soil background. To accurately estimate the true canopy reflectance, it is desirable to extract the canopy reflectance component from the mixed signal. This study used a system equipped with a RGB digital camera and a spectrometer to study the spectral mixing problem under outdoor lighting conditions. It was found that the shadow effects, which have been ignored in many agriculture applications, must be considered under sunny conditions when doing spectral unmixing. The green and near infrared (NIR) peaks in the mixture reflectance due to vegetation were successfully simulated under different canopy coverage. The residual error in the NIR wavelength range indicates that vegetation tends to be over estimated in the simulated reflectance. Finally, in an unmixing process, the canopy reflectance caused by the vegetative component of the mixed reflectance signal was successfully extracted.