Parts of the Dock background (at least the "plain" one mentioned in the linked question) are stored in separate files with all other Dock resources in
If your Dock is on the left, check out the image files whose names start with left, the separator between applications and documents is in the files with names starting with separator, etc.
Unfortunately, it appears that the yellow area is the extend of what can be changed by editing these files.
There doesn't seem to be a "center" image. Some people even resort to patching the actual Dock program (you should have a backup before doing this, just in case), as the color information now seems to be part of the Dock's code.