There is a standardized internet color palette that will ensure that all of your colors show on all platforms with at least 256 colors. It's called the Netscape palette, but it goes for all browsers, on the Mac and PC. There are 216 valid colors.
Basically, for HTML colors, you are allowed any combination of:
So 0033cc is valid. This prevents some 256 color systems from dithering to attempt to make up the color you want. Usually, the dithering is very bad. In other words, say there's 500 pixels accross, and the color you want is c0c0c0 - a medium grey. The result is that the first few pixels are cccccc and then there's an oddball of 999999. This works out mathmatically to get an average of c0c0c0, but math and human perception are two different things.
I am also guilty of skipping the 216 color thing, but I am slowly correcting all colors to the nearest palette entry.
Another good idea is to reduce graphics to 216 colors. In Paint Shop Pro, open up this graphic:
and click on Image --> save palette. Then open up your graphics and set them to this palette.
When you reduce colors in your graphic editor, it will dither as well, but it will use a smarter dithering. It will look a thousand times better than letting Windows (or Mac OS) dither it cheaply. And all users, no matter how cheap their video card, will see the graphic as it was intended.
Different editors will vary, but should be similar. All systems of at least 256 colors will have these 216 colors in their system palette.
-- yes, Virginia, there are standards in the www! --