Use websafe colors
Websafe colors are
colors that are common to both Mac OS and Windows platforms. These
colors are not dithered when viewed in a web browser on a computer
display set to 256 colors. For a Fireworks PNG file, changing colors
to websafe in the Optimize panel affects only the exported version
of the image, not the actual image.
To Force all colors to be websafe colors, select Web
216 from the Indexed Palette pop-up menu in the Optimize panel.
To create an adaptive palette that favors websafe colors,
select Web Adaptive from the Indexed Palette pop-up menu in the
Optimize panel.
To Force a color to its closest websafe equivalent, select
a color in the Optimize panel color table, and then click the Snap
To Web Safe button 
.
PDF (adobe.com)
