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The 16 million possible colors is because images using the RGB system (en.wikipedia.org/wiki/RGB_color_model). So each color is a combination of Red (256 possible variants), Green (256 possible variants) and Blue (256 possible variants). 256 * 256 * 256 = 16 777 216.
Your dataset consists of only 649 instances, a neural network requires more data to train. You should consider using different methods for analyzing this dataset, for instance linear regression, decision tree, random forrest, XGBoost, KNN...
