Web Design in a Nutshell, 3rd Edition

Highcolor (15- or 16-bit)

Highcolor systems are capable of displaying thousands of colors. The most popular variation of Highcolor monitor is 16 bit, which assigns 5 bits of data to the red channel, 6 bits to green (because the human eye can discern more shades of green), and 5 bits to blue. This is often referred to as the 565 model. If you do the math, that’s 32 × 32 × 64 for a total of 65,536, or “thousands of colors.”

15-bit monitors use a 555 model, with 5 bits of data assigned to each color channel, resulting in 32,768 colors. 15-bit monitors are extremely rare these days, so this section focuses on Highcolor in 16-bit monitors.

It is important to understand that the 16-bit high color spectrum is fundamentally different from 24-bit color. It is not merely a subset of the colors in the Truecolor space. It is an entirely different set of colors. To better understand, consider just the red color channel. In 24-bit color, the range of shades from 0% (black) to 100% (white) is divided into 256 increments. In 16-bit Highcolor, the range of shades from black to white is divided into 32 increments. Aside from black and white, the shades on the two scales do not coincide; they are always slightly different. Apply this across all three color channels and it should be clear how you get a completely different set of colors (at least mathematically) on 16-bit monitors.

What this means for web designers is that whatever color you specify by RGB color values on a scale from 1 to 255 (as is ...