RGB888 to RGB565 pins for an AL250
RGB888 representation uses more bits to represent your image. This means that your image will have a better color resolution. If you want to convert this data to RGB565 you will be effectively reducing your color resolution. This can be done by using the mapping in the datasheet you provided. The pin mapping provided in the datasheet is done in such a way that the bits with the highest significance from RGB888 are used and the lower significance bits are discarded.