NES does not use an RGB encoding.
As I have read, the actual encoding is based on analog data.
Especially on NTSC consoles, which is why even the real hardware could display different colors on different TVs.
The reason people said "divisible by 8" is because SNES uses five bits per channel, for a total of 32 intensities each of red/green/blue.
Conversion from standard 24 bit (8 bit, or 256 intensities per channel) = 256/32 = 8.
Though for more accurate conversion from SNES 15-bit to RGB 24-bit, I think byuu recommended duplicate the lowest three bits, to retain proper white and black values. I can't remember for sure.