Drum notation and style

[YTG 1234]

specifying the color of the bar lines that connect staves and the 
entire opening bar line

Just looking through the list of layout objects in the internals 
documentation, I think you need to override the color of SpanBar and 
SystemStartBar, like

%%%

\override StaffGroup.SpanBar.color = "slateblue"
\override Score.SystemStartBar.color = "slateblue"

%%%

SystemStartBar only works for me when overriding in the Score context.