What's Color depth or Bit Depth? for mid-term exam

Color depth or bit depth, is a computer graphics term describing the number of bits used to represent the color of a single pixel in a bitmapped image or video frame buffer. This concept is also known as bits per pixel (bpp), particularly when specified along with the number of bits used. Higher color depth gives a broader range of distinct colors.
Color depth is only one aspect of color representation (formally, the gamut: which colors can be expressed), expressing how finely levels of color can be expressed (formally, gamut depth); the other aspect is how broad a range of colors can be expressed. The RGB color model, as used below, cannot express many colors, notably saturated colors such as yellow. Thus, the issue of color representation is not simply "sufficient color depth" but also "broad enough gamut".
from http://en.wikipedia.org/wiki/Color_depth

「位元」 是二進制位的簡稱，是最小的單位來處理電腦的數據資料。一個單位元能處理兩個數值的其中之一(0或1)，我們必須把位元繼續結合成為更大的單位，稱為「位元組」，以成為更有意義的數據分析，一個位元組能容納八個位元。 位元深度也稱為像素深度或色彩深度，是用來度量影像中每個像素有多少色彩資訊可用來顯示或列印。較大的位元深度 (每像素較多資訊位元) 表示數位影像中具有較多的可用顏色，而色彩表現也較正確。例如： 黑白：影像中的每一點，又稱為像素(pixel)，只佔1 bit(0或1)。 灰階：影像中包含深淺不同的灰色，每個像素佔8 bits，每點有 2^8=256 種變化。 16色：影像中最多用到 16 種顏色，每個像素佔4 bits，每點有 2^4=16 種變化。 256色：影像中最多用到 256 種顏色，每個像素佔8 bits，每點有 2^8=256 種變化。 全彩：影像中最多用到 1677 萬種顏色，每個像素佔24 bits，每點有 2^24 = 1677 萬種變化。
from http://www.idsvision.com.tw/phpbb2/viewtopic.php?t=447