16-bit graphics mode (also known as 'high color' or 'thousands of colors') is truly an oddity when start to look at how one pixel is represented.
Try dividing 16 by 3. (That is, dividing sixteen bits into Red, Green, and Blue values.) It can't be done. Ack! I have a leftover bit!
In true 16-bit mode, the extra bit is allotted to green, because the human eye is most sensitive to green. Here's a binary representation of a 16-bit pixel:
1111100000011111
| | \Blue (five bits)
| \Green (SIX bits!)
\Red (five bits)
This gives us a total of 32 shades of red, 64 shades of green, and 32 shades of blue, for a total of 65536 possible colors.
16-bit mode is also referred to as '565' mode in reference to the number of bits allotted for each color channel.
Compare 16-bit graphics with 15-bit graphics; the two are very similar but most graphics routines are annoyingly incompatible. Remember: for 16-bit graphics, you must shift the green channel SIX to the left!