I see what you are talking about. On my machine (Vista Business 32-bit, FF3.5) it seems that the yellowness present during the decay is due to the ClearType combination trying to work on alpha mixed text and that you fade the text through a bluish background. I know white + blue! = Yellow, but no matter what FF does under the hood when rendering transparent text, in combination with your OS, as well as a basilion of other things makes it yellow.
When I turn off ClearType on my machine, the yellowish one seems to disappear, or at least is much less noticeable than with it.
Iβm afraid you wonβt be able to control this with JavaScript code, but I might try what others suggested: keep the text visible, but link it to blue, which matches the background to white.
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