Most web images are created by designers using 24‑bit color displays (which display over 16 million colors), although some users view web pages on computers with 8‑bit color displays (which display only 256 colors). As a result, web images often contain colors not available on some computers. Computers use a technique called dithering to simulate colors they can’t display. Dithering uses adjacent pixels of different colors to give the appearance of a third color. For example, a red color and a yellow color may dither in a mosaic pattern to produce the illusion of an orange color that the 8‑bit color panel doesn’t contain.
Use colors in the Web panel to ensure that colors won’t dither when displayed in Windows or Mac OS systems capable of displaying 256 colors. When creating an original image, you can use the Color Picker to choose web‑safe colors.
When optimizing images, keep in mind that two kinds of dithering can occur:
Occurs in GIF and PNG‑8 images when Photoshop Elements attempts to simulate colors that aren’t in the current color table. You can control application dither by choosing a dithering pattern, or you can try to avoid application dither by adding more colors to the table.
Occurs when a web browser using an 8‑bit color display (256‑color mode) attempts to simulate colors that aren’t in the 8‑bit color panel. Browser dither can occur with GIF, PNG, or JPEG images. In Photoshop Elements, you can control the amount of browser dither by shifting selected colors in the image to web‑safe colors. You can also specify web‑safe colors when choosing a color in the Color Picker.
You can preview application dither in GIF and PNG‑8 images. Images with primarily solid colors may work well without dithering. Conversely, images with continuous-tone color (especially color gradients) may require dithering to prevent color banding.
You can preview browser dither directly in Photoshop Elements or in a browser that uses an 8‑bit color display (256‑color mode).
Set your computer’s color display to 8‑bit color (256 colors). See your operating system’s documentation for information on changing the color display.
Select a browser from the Preview In menu in the Save For Web dialog box.