1. A pixel color conversion method applied to an output device, comprising: providing a trainable database for storing a plurality of data values corresponding to a set of indexes for use performing pixel color conversion; regarding at least one source image, training the trainable database, comprising: within the trainable database, storing a set of source samples as the set of indexes, wherein the set of source samples represent pixel colors of the source image; within the trainable database, storing a plurality of sets of candidate data values, a plurality of candidate data values in each set of candidate data values respectively corresponding to a plurality of indexes in the set of indexes; converting the set of source samples into a plurality of sets of target samples according to the plurality of sets of candidate data values, respectively, wherein each set of target samples represent pixel colors of a target image; utilizing the output device to output each set of target samples to generate output results of the sets of target samples; and according to comparison results between the output results of the sets of target samples outputted by the output device, selecting a set of candidate data values from the candidate data values stored in the trainable database as the plurality of data values corresponding to the set of indexes; and according to the plurality of data values corresponding to the set of indexes in the trainable database, converting source samples of another source image into target samples for being outputted by the output device.