When a computer monitor is set to display 256 colors, web browsers will select from a palette of only 216 colors to display any part of the page that requires color. This palette is called a "cube" because it measures values in three dimensions: Red, Green, and Blue. If your page includes colors that aren't one of the 216 colors, the browser "dithers" colors together to approximate your color. Sometimes this looks OK, ofttimes not.

DynaKewb is a page that allows you to see the 216 colors on a palette swatch, and to dynamically change the background of the page as you pass the cursor over each color. You can lock in a color by clicking on a choice, and unlock it by clicking again. The code for the color shows up in a text box on the page, so you can copy the value and use it in the HTML of your own page.