Change sidebar background colour?

AFAIK, what you're looking to do is impossible without using Shapeshifter.

 

Well. It ain't never pretty, is it?

In theory it should still be possible. Basic ShapeShifter things, such as modifying the Apple logo and menubar, etc, etc can be done without ShapeShifter using a few .rsrc files. However, the sidebar color is not stored in the resource file, and this more advanced stuff is where ShapeShifter comes in handy as it injects itself into memory space to modify pre-existing areas.

In 10.3, I remember a macosxhint stating how to color the sidebar by modifying the Finder binary. In 10.4, of course, Finder has changed and the location of that address has as well. Back when I upgraded to 10.4, I tried looking through the Finder binary to recolor the sidebar and never have found the particular address.

I've been too lazy and unwilling to actually do a thorough investigation, however, and I'm sure it's still indeed possible using said techniques. However, Apple may have truly embedded some of these things, as 10.4 has been out for so long, and I'd have thought someone would have found/published it in a clean utility by now.