Length being a multiple of 2 makes sense. Rgb565 means 2bytes per RLE value. Data stream is half values half lengths. So every datum is 3 bytes. So it makes sense to send number of half words. Worst case: you overlong by one byte. NBD. The alternative means sending one byte extra ALWAYS to have enough bits for length.
Improvement would be to send number of RLE datas (number of 3 byte quantities)