Actually we do not use a background color for the fieldset, but a background image with background position top. We do not use background color because it is impossible with the current rendering of the skinned fieldsets. The background image we apply is a semi-transparent png file, and as you know, IE6 does not support the alpha channel of .png. Of course, there are a couple of fixes, however none of them can be applied to an element with background-position. Knowing this, you can either:
1. Disregard IE6, as it is not a major browser anymore
2. Entirely remove the background image with CSS for IE6 using a browser-specific hack. For your convenience, you may use this one:
_background: none !important;
This is all that can be done with IE6, and we are not planning further fixes.
the Telerik team