There are 140 color names are defined in the HTML and CSS color specification. The 17 standard colors are aqua, black, blue, fuchsia, gray, green, lime, maroon, navy, olive, orange, purple, red, silver, teal, white, and yellow. The extended color set includes 123 more colors. One way to define colors in HTML is using the "RGB" method which specifies the amount of Red, Green and Blue in a Hexadecimal value between X'00' and X'FF' which is a decimal value between 0 and 255. The three values are then used determine the color's exact shade.
For example, the color red would have a red value of X'FF' and would have no value for the green and the blue, so to specify red in this RGB format, it is #ff0000. To make the red darker, just lower the red value while keeping the green and blue value set at zero. So, a maroon color would be #800000 and a very dark red would be #400000. To make magenta, which is a mix of equal parts red and blue, specify #ff00ff and a purple which is darker than magenta would be #800080.
Using this method allows for over 16 million different colors. HTML allows user's to specify one of 140 color names if the user chooses not to use the hexadecimal values when coding. The table below lists those 140 colors along with the RGB hexadecimal value.
There are three website which I really like to help with web color selection, the first of which allows you to link to a picture and the website will find the perfect matching colors: