16-bit High Color
When all 16 bits are used, one of the components (usually green, see below) gets an extra bit, allowing 64 levels of intensity for that component, and a total of 65,536 available colors.
Sample layout of real 16 bit color data in a 16 bit pixel (in RGBAX notation)
This can lead to small discrepancies in encoding, e.g. when one wishes to encode the 24-bit color RGB (40, 40, 40) with 16 bits (a problem common to subsampling). Forty in binary is 00101000. The red and blue channels will take the five most significant bits, and will have a value of 00101, or 5 on a scale from 0 to 31 (16.1%). The green channel, with six bits of precision, will have a binary value of 001010, or 10 on a scale from 0 to 63 (15.9%). Because of this, the color RGB (40, 40, 40) will have a slight purple (magenta) tinge when displayed in 16 bits. Note that 40 on a scale from 0 to 255 is 15.7%.
Green is usually chosen for the extra bit in 16 bits because the human eye has its highest sensitivity for green shades. For a demonstration, look closely at the following picture (note: this will work only on monitors displaying true color, i.e., 24 or 32 bits) where dark shades of red, green and blue are shown using 128 levels of intensities for each component (7 bits). Readers with normal vision should see the individual shades of green relatively easily, while the shades of red should be difficult to see, and the shades of blue are likely indistinguishable. More rarely, some systems support having the extra bit of color depth on the red or blue channel, usually in applications where that color is more prevalent (photographing of skin tones or skies, for example).
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