HTML and CSS Reference
In-Depth Information
Figure 3-9
Color addition in the RGB color model
Varying the intensity of the three primary colors creates other colors. Orange, for
example, is created from a high intensity of red, a moderate intensity of green, and a
total absence of blue. CSS represents these intensities mathematically as a set of numbers
called an RGB triplet , which has the format
rgb( red , green , blue )
where red , green , and blue are the intensities of the red, green, and blue components
of the color. Intensity values range from 0 (absence of color) to 255 (maximum intensity);
thus, the color white has the color value rgb(255, 255, 255) , indicating that red,
green, and blue are mixed equally at the highest intensity, and orange is represented
by rgb(255, 165, 0) . RGB triplets can describe 256 3 (16.7 million) possible colors,
which is a greater number of colors than the human eye can distinguish. You can also
enter each component value as a percentage, with 100% representing the highest inten-
sity. In this form, you would specify the color orange with the following values:
rgb(100%, 65%, 0%)
The percentage form is less commonly used than RGB values, but many page design-
ers fi nd it easier to work with.
CSS also allows RGB values to be entered as hexadecimal numbers. A hexadecimal
number is a number expressed in the base 16 numbering system rather than in the com-
monly used base 10 system. In base 10 counting, numeric values are expressed using
combinations of 10 characters (0 through 9); hexadecimal numbering includes six extra
characters: A (for 10), B (for 11), C (for 12), D (for 13), E (for 14), and F (for 15). For values
above 15, you use a combination of those 16 characters. For example, 16 has a hexa-
decimal representation of 10, and a value of 255 is represented as FF in hexadecimal
numbering. The style value for color represented as a hexadecimal number has the form
# redgreenblue
where red , green , and blue are the hexadecimal values of the red, green, and blue
components. Therefore, the color yellow could be represented either by the RGB triplet
rgb(255,255,0)