From Designers File to Web Build File: Solving Special Transparency Problems with Essential Layer and Layer Mask Tricks
The Obvious Solution (or So It Would Seem)
At first glance, you might think that the obvious solution to this dilemma would be to abandon the idea of a transparent GIF altogether and save the entire block as a JPEG. The file size would probably be smaller and we wouldn't have to worry about losing the blue effect when the background visibility was turned off to create the GIF. The problem with this approach, however, is that when the image is saved as a JPEG, the blue background color (#336699) changes ever so slightly so that when displayed in a browser, it does not match the same color when it is called out in the HTML code. The same thing happens if you save the file as a GIF with transparency turned off, although it is not nearly as noticeable as with the JPEG. As you would expect, this slight color difference is more noticeable in High Color (thousands) than it is in True Color (millions) display mode. It is also more noticeable on PCs than it is on Macs, and the amount of difference varies depending on the actual color you are using. (See Figure 4.)
Figure 4 The visible difference when using color called out as HTML and using color in a JPEG.
So, with those two solutions disqualified, a transparent GIF is the final thing to try.