I spent some time on coding a small graphical interface on a TFT screen. In short, it's about transmitting pixels coded as 16-bit frames, from the MCU to the graphical controller.
Over time I just got tired of looking for an online service to convert 24-bit color codes to 565 format code and I built this small converter. As the 565 format is actually a palette of the 24-bit format (1 byte of data is lost during the conversion), be aware that each 565 code has several equivalents in the 24-bit format.
Here are the conversion rules for 16-bit to 24-bit...
R8 = (R5 * 527 + 23) >> 6; G8 = (G6 * 259 + 33) >> 6; B8 = (B5 * 527 + 23) >> 6;
...and for 24-bit to 16-bit :
R5 = (R8 * 249 + 1014) >> 11; G6 = (G8 * 253 + 505) >> 10; B5 = (B8 * 249 + 1014) >> 11;
Simple. Practical. And you can get it here.