The problem is caused by an infamous IE bug - the alpha channels of 24-bit PNGs are rendered with a gray matte. There are many work-arounds of the problem, and people have already written some automatic solutions to fix it.
The cleanest solution to the problem is to adjust your CSS like suggested in the following article
. Of course, you can choose to use an automated solution, like the IE PNG Fix
- it could, however, introduce performance problems.
the Telerik team