iPhone 3GS has color depth of 24-bit color or 18-bit color?
i found the answer, iPhone 3Gs use 24-bit Emulation color not the 24-bit true color.


  18-bit:  Almost all of the least expensive LCD displays (such as typical
  twisted nematic types) provide 18-bit color (64 × 64 × 64 = 262,144
  combinations) to achieve faster color transition times, and use either
  dithering or frame rate control to approximate 24-bit-per-pixel true
  color,[12] or throw away 6 bits of color information entirely. More
  expensive LCD displays (typically IPS) can display 24-bit or greater
  color depth.


http://en.wikipedia.org/wiki/24-bit_color#True_color_.2824-bit.29

Note: 18-bit color display was used either dithering or frame rate control to simulate 24 bit (16.76 million colors) per pixel true color; yielding sub-optimal color accuracy and often having poor viewing angles (http://techreport.com/review/23204/tr-summer-2012-system-guide/14)

So correct answer is 24-bit emulation color.