This is something I've just spotted, and is actually very subtle, but now I've spotted it, is bugging me.
Basically have a site here:
The various headings, eg The Safari Awards 2011 were created in FW, and exported out as JPGs (they might actually be OK as GIFs, but have done JPGs because of the gradient fill of the text, and drop shadow).
Anyway - the canvas color pre export is #39434A. Post export its changing to #39444A.
Is there any reason this should be happening, or any way to prevent it from happening?
One thing you can try doing is setting the Matte to the canvas color in the Optimize panel.
Yes, there's a reason it's happening. The JPEG format is a mathematical representation of the data in an image. The compression works by throwing out what are called "higher order terms." The higher the compression, the more detailed information is lost. Color fidelity is one thing you can lose and it's usually more noticable with line art. If you have sharp, high-contrast lines, you'll lose crispness along them, as high contrast is also represented by "higher order terms." (The .jpg format is really designed for photos, JPEG stands for Joint Photographic Experts Group.)
Things you can try. First, try saving at a higher quality .jpg. Compare that to a .gif. Depending on the art, the colors lost in a .gif may not be as noticable to you as the color shift in the .jpg. Also try a PNG24, as these are full color, but not lossy. Compare file sizes and see which image gives you acceptable quality for the file size.