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
palette doesn’t contain.

Dithering simulates continuous tones

Use colors in the Web palette 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:
- Application dither
-
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.
- Browser dither
-
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
palette. 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.