It depends on which theme. The Halo theme has a trackColors style (plural, not singular). The Spark skins for halo don't have any styles right now.
I think you reference the custom skins in the declaration referenced by vertcialScrollBarStyleName
Flex SDK Developer
Adobe Systems Inc.
Depending on how you're trying to style, you can set the horizontalScrollBarStyle, verticalScrollBarStyle, and the baseColor style, as seen in the following example: http://blog.flexexamples.com/2009/09/27/setting-the-base-color-on-the-nested-scrollbars-in -the-halo-datagrid-control-in-flex-4/
Thank you Peter and Alex for the replies.
Setting the baseColor in the stylesheet worked (I was trying everything else but).
It would be nice to have greater control over the track and trackbar colors when using the Spark theme, my datagrid header background is dark with a fontcolor of white and a symbolcolor of white, so trying to match a dark scrollbar basecolor the trackbar is almost lost in a dark track.