I must begin by giving the proper credit where it is due: I found this hint by way of GUI gurus demograph68 and Dave-o from the MacNN forums. Now then, if you're tired of having a plain white sidebar in your Finder windows, you can change the color to any you wish.
Note: This procedure involves modifying files crucial to the OS and, if done incorrectly, can basically hose your Finder!. So, back up the files to be changed before you change them. Also note that this procedure works only for 10.3.5, and may break with future system updates. Read the rest of the hint for the detailed how-to...
[robg adds: I tested this one, and it indeed works, as you can see in the image at left (now I just need to go pick a color I really want to use!). So any hotshot Cocoa authors out there want to create a GUI for this hack? Not knowing the first thing about Cocoa, I have no idea if it's even possible to modify the Finder on the fly, nor if it's "safe" to do so ... but it'd be a cool hack!]
Navigate to System -> Library -> CoreServices -> Finder -> Contents -> MacOS -> Finder. You will need to Control-click and select Show Package Contents on the Finder item to get to the Contents folder). Now copy the Finder file to a safe place and keep it as your backup. Make another copy of the Finder on your desktop, and open this Finder copy in HexEdit, press Command-J, and type in 0029EB10 then click GO. You will see this:

Open the DigitalColor Meter application inside the Utilities folder and set it to display "RGB as Hex Value, 16-bit." Mouse around and find the color you wish to use. Hit Shift-Command-H to "lock" the color you'd like to use in DigitalColor Meter, and make note of the RGB address that it displays. For example, say you wanted to use [R 8888] [G A4A4] [B B9B9] for your sidebar color. Switch back to HexEdit, and first highlight (via click-and-drag) the six "FF" values (which represent white). Next, type in your chosen value in pairs. For this example, that would be 88 88 A4 A4 B9 B9. Make sure you are replacing the existing values and not adding new entries! Now save this modified file to the desktop.
Go back to System -> Library -> CoreServices -> Finder -> Contents -> MacOS -> Finder, and move the original Finder file to the trash. You will need to have an administrator account, and will be prompted for your password. Now move your modified Finder file to the system folder (again, you'll be prompted for your password). Be sure the file is named Finder, just as the original. Again, you will need to have an administrator account and will be prompted for your password.
Now log out and in again. If all goes well you will now see your chosen color in the sidebar! Good Luck!
[robg adds again: You can skip the logout/login with a simple Finder restart; sudo killall Finder in the Terminal will do it...]

