Published alpha tween different from .fla

I have read many similar issues but cannot find a solution.
Just starting with flash so I’m making a simple gallery. I’ve added a background layer where I change the background color using large rectangles. At certain parts of the gallery I have an alpha tween for the background 15% to 100%. Works fine when I test the movie but when it’s published on the site it has a much subtler effect. It almost seems like the default white background is actually dark for some reason on the swf so the tween kind of has a reverse effect.
Is there a common mistake where the background is not correct when publishing.