I don't know where this myth that the N900 displays 16M colors came from. No matter what the screen hardware can display (18bit maybe), the OS runs in 16 bit mode.
Yes, you'll notice a big difference between 16bit and 24bit when looking at undithered color gradients. But dithering, especially at the high resolution of the N900, makes it less noticable.
BTW, 32 bit colors is the same as 24 bit colors, with the additional bit unused or used for storing a transparency value. Both display 16M colors.