To elaborate on what's just been said, the reason why you don't get 255 in 16 bit graphics mode is because only the highest bits are stored (5 or 6 depending on the card). 255 requires 8 bits.
255 = 11111111
248 = 11111000
252 = 11111100
The most common 16 bit mode is 565, which is 5 for red, 6 for green and 5 for blue: 5 + 6 + 5 = 16 bits.
24 and 32 bit graphics modes use all 8 bits to store each element (32 bit mode has an extra alpha channel as well as the normal red, green and blue channels).
255 = 11111111
248 = 11111000
252 = 11111100
The most common 16 bit mode is 565, which is 5 for red, 6 for green and 5 for blue: 5 + 6 + 5 = 16 bits.
24 and 32 bit graphics modes use all 8 bits to store each element (32 bit mode has an extra alpha channel as well as the normal red, green and blue channels).