See this example: http://jsfiddle.net/j6Udd/1/
If you hover over the series 1, you'll see tooltip border is the same as series color, then move to series 2 tooltip's border changes properly, then hover series 1 again. You'll see that tooltip border still has previous series alpha value.
Related forum thread: http://highslide.com/forum/viewtopic.php?f=9&t=11100
Partial fix for bug #284; Series rgba color and tooltip border
Note that the previous commit does not take care of this issue in VML case only SVG
Fixed VML case of Series rgba color and tooltip border Closes #284