Could be a PNG issue
Some browsers (guess which?), while they support PNG, don't properly handle PNG transparency. You'll see a slight darkening of the underlying color, so it would look like the effect you're describing.
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Microsoft offers them the one thing most business people will pay any price for - the ability to say "we had no choice - everyone's doing it that way." -- [link|http://z.iwethey.org/forums/render/content/show?contentid=38978|Andrew Grygus]