Using dither

The Use Dither (8-bit/channel images) option controls whether to dither colors when converting 8-bit-per-channel images between color spaces. This option is available only when the Color Settings dialog box is in Advanced Mode. When the Use Dither option is selected, Photoshop mixes colors in the destination color space to simulate a missing color that existed in the source space. Although dithering helps to reduce the blocky or banded appearance of an image, it may also result in larger file sizes when images are compressed for Web use.