The Playground

According to Elizabeth Castro, you can use any colours in you web design that you like. Here's an excerpt from Elizabeth's book, HTML, XHTML, and CSS, Sixth Edition: Visual QuickStart Guide:

Currently (mid 2006), around 80% of the Web surfing public use 24- or 32-bit monitors, 16% use 16-bit monitors, and fewer than 1% use 8-bit monitors. In 2002, those numbers were quite different: 50% on 24-bit, 40% on 16-bit and 10% on 8-bit. The trend is clearly toward so-called “True Color” monitors in which any of 16 million colors can be displayed. In the days when 8-bit monitors were the norm, you had to restrict yourself to only the browser safe colors in order to ensure that the colors you chose would appear correctly on your Web pages. However, as the numbers show, so few people are using 8-bit monitors, that it is no longer an issue. You may use any colors you like.

The only reason to use the browser-safe palette is if you have a concern that your web design work will be viewed from a 256 color (8-bit) computer system. Only 216 colours were considered safe for web browsers. Browser-safe colours can only contain 00, 33, 66, 99, CC, and FF values in each of the paired digits of the RGB spectrum.

The word from Lynda.com (who are credited with the distinction of having first identified and published the web safe colour palette) is that...

There may be resurgence in the need for the browser-safe palette when designing for alternative online publishing devices, such as cell phones and PDAs. Those systems are still in 1-bit (black and white) or 8-bit color. Right now, very few people are designing their web sites to work on those systems, so the need for the browser-safe color palette is definitely downgraded to a mere shadow of its former glory.

Below is a table of the browser-safe colours: